wwan: T9350: query ModemManager instead of the QMI control port

show_wwan.py drove qmicli against /dev/cdc-wdmN, which assumes the modem
exposes a QMI control port. An MBIM modem does not, so every subcommand of
"show interfaces wwan wwanN" failed on one - after waiting out a five second
transaction timeout, twice over for "signal":

  $ show interfaces wwan wwan0 signal
  Command not supported by Modem                       (10.6s)

  $ qmicli --device=/dev/cdc-wdm0 --device-open-proxy --nas-get-signal-info
  error: couldn't create client for the 'nas' service: CID allocation failed
  in the CTL client: Transaction timed out

  ports: cdc-wdm0 (mbim), ttyUSB0 (at), wwan0 (net)

ModemManager has already probed the modem and speaks whichever protocol it
exposes - QMI, MBIM or plain AT. It is a hard dependency of "interfaces wwan"
anyway, as conf_mode cannot dial without it, and it answers from its own cache
over D-Bus without touching the device:

  $ show interfaces wwan wwan0 signal
  Access technology: lte                               (0.2s)
  Signal quality:    58%
  Operator:          Telekom.de
  Registration:      home
  Packet service:    attached
  State:             connected

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