bearer-mbim: session id 0 by default in all non-multiplexed connections

The original logic in the MBIM modem would assume that if we had more than one network interface in the same modem, we could connect multiple data interfaces, each one with a different session. That logic is actually wrong, when using the master (non-multiplexed) network interface we should always use session id 0, which is the one holding all non-VLAN-tagged traffic.

So, remove the logic that automatically creates new bearers with a different session id, as that is really wrong.

(cherry picked from commit e5305498)