MM was not updating the EPS registration status for qmi modems. This
led to LTE-only modems never having 'registered' status. This was
happening for Quectel EC21-V modem.
The workarounds for Sierra HL7588 were executed only for Sierra VID/PID.
Make sure to apply them also for rebranded Dell modem with VID=0x1519
and PID=0x0443. An additional twist is needed, as Dell reused same
VID/PID for a Telit modem, so we additionally check the ouput of
AT+CGMM.
Previously, probing or not the commands depended on a race condition,
and, besides, we need the AT ports for implementing workarounds for
Sierra HL7588. Therefore, we make sure we always probe the AT ports in
the Dell plugin, even when an MBIM port has already been detected.