broadband-modem-mbim: ignore slot status reports during a slot switch action

When we explicitly request to perform a SIM slot switch operation, we should ignore the 'slot info status' indications, or otherwise they'll end up interfering with the correct reporting of the slot switch operation.

We could have attempted to complete the slot switch operation early using the 'slot info status' indications, but that solution would have forced us to assume that the info in the indications is the one we had requested, which is not easy because the indication is reported once for each slot, so a single SIM slot switch operation triggers multiple 'slot info status' indications, once per slot involved (one switches to active, the other one switches to inactive).

Fix suggested by Pavan Holla pholla@chromium.org.

Fixes #550 (closed)