[review] Use AT+CESQ for signal interface in MBIM modems if TTY available

[Aleksander Morgado]

Hey,

>
> This MR enables support for the "Signal" interface in MBIM modems if
> they expose an AT-capable TTY port and AT+CESQ is supported. If no AT
> port is available in the device, the modem will report no extended
> signal capabilities, as it was doing until now for all MBIM modems.
>
> https://gitlab.freedesktop.org/mobile-broadband/ModemManager/merge_requests/18
>

I've closed this MR because I think using the MBIM AT&T extensions
service to get the same information fits much better in the MBIM modem
object:
https://gitlab.freedesktop.org/mobile-broadband/ModemManager/merge_requests/25

-- 
Aleksander
https://aleksander.es