broadband-modem-qmi: enable/disable messaging AT unsolicited events too

When the host is resuming from system suspend, QMI indications sent by the modem at resume time can be lost. The exact reason why it happens is still unknown. Until this is fixed, ModemManager currently workarounds that in QMI mode by listening and reacting to AT URCs too, which are being received reliably. In order to achieve that, messaging_setup_unsolicited_events chains the parent's implementation with its own, effectively setting up handlers for both AT and QMI channels.

This worked fine on modems such as EG25 which enable SMS indications by default. However, some modems, such as BM818, don't have these indications enabled on boot and don't report incoming messages via AT unless requested via AT+CNMI.

To make SMS handling on resume reliable on such modems, make sure that MMBroadbandModemQmi also enables/disables unsolicited events in the same way it already sets up handlers for them.