[feature] Support hardware model for Zyxel NWA50AX & NWA55AXE

[FrankGiesecke]

Is your feature request related to a problem? Please describe.
As the hardware model of my devices is not listed in the upload dialog, I could not upload my firmware files using the UI or API.

Describe the solution you'd like
I'd like to upload custom firmware files for my Zyxel devices using the API.

Describe alternatives you've considered

Additional context

[nemesifier]

Here:
https://github.com/openwisp/openwisp-firmware-upgrader/blob/master/openwisp_firmware_upgrader/hardware.py