Network config > Edit phyiscal > Save button inactive

## Blocking bug

Currently, I'm not able to deploy any new nodes (VMs on Hyper-V server) any more with a configured static IP.

When I edit the network configuration at http://maas-server.maas:5240/MAAS/l/machine/abcdef?area=network by clicking "Edit Physical...", the save button is deactivated and the field "Link speed" is marked with an error. But the save button is required when I want to save the configured static IP address.

Pre-filled values:
* Link speed: 1
* Interface speed: 0

The error message is: "Error: Link speed cannot be higher than interface speed"

## Expected behaviour

1. If the machine booted successfully from that interface,
  * by any logic, the interface speed must always be something > 0, at least it must be >= the detected link speed
  * the check for "Link speed cannot be higher than interface speed" should be disabled or the error should be a warning only, so that saving is possible in any case
2. It should be possible to save network configuration (like static IP) for not-connected network adapters, also when there is a warning regarding the link/interface speed - it should show the warning but it shouldn't block saving the configuration

## Tried workarounds

1. Manually set field interface speed to 1 >> not working
  * Link speed: 1
  * Interface speed: 1
2. Manually set field link speed to >> not working
  * Link speed: 0
  * Interface speed: 0
3. Delete the machine from MAAS config completely and re-commission from scratch >> not working

## Environment

There is only 1 network adapter and the machine already booted successfully from PXE with this network adapter, so there is no link-down issue or anything else. Commissioning was successful without errors.

MAAS version: 2.9.2 (9165-g.b5dc1fd6c) on Ubuntu 20.04

## Detected environment

This is the log from lshw for the network adapter:
----SNIP----
      - lshw:node:
        id: network
        claimed: true
        class: network
        modalias: vmbus:635161f83edfc546913ff2d2f965ed0e
        - lshw:description:
          Ethernet interface
        - lshw:physid:
          1
        - lshw:logicalname:
          eth0
        - lshw:serial:
          00:15:5d:08:34:44
        - lshw:size:
          units: bit/s
          1000000000
        - lshw:configuration:
          - lshw:setting:
            id: autonegotiation
            value: off
          - lshw:setting:
            id: broadcast
            value: yes
          - lshw:setting:
            id: driver
            value: hv_netvsc
          - lshw:setting:
            id: duplex
            value: full
          - lshw:setting:
            id: firmware
            value: N/A
          - lshw:setting:
            id: ip
            value: 192.168.36.213
          - lshw:setting:
            id: link
            value: yes
          - lshw:setting:
            id: multicast
            value: yes
          - lshw:setting:
            id: speed
            value: 1Gbit/s
        - lshw:capabilities:
          - lshw:capability:
            id: ethernet
          - lshw:capability:
            id: physical
            Physical interface
        - lshw:hints:
          - lshw:hint:
            name: icon
            value: network
----/SNIP----