base-modem: allow looking for the QMI port associated to a given data port

QMI and wwan ports come in pairs. Each wwan port has an associated control QMI
port, which is the only port allowed to send the Start|Stop Network QMI requests
to start|stop the connection in the given wwan interface.

Paired QMI and wwan interfaces (should) share the same parent udev device,
quoting Bjørn:

    "If we ignore the unfortunate 3.4 and 3.5 kernels, then a matching wwanX
     and cdc-wdmY set will always share the same parent USB interface on QMI
     devices.

     Having the same parent USB device is *not* sufficient.  You cannot control
     wwan0 using cdc-wdm1 in the above example."

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