modem: cmux: Simplify resync mechanism

Some modems don't start sending resync flags as described in
3G TS 27.010 V2.0.0 5.2.5, which results in the CMUX being
stuck in resync mode for said modems.

This patch simplifies the resync mechanism to simply drop
invalid frames, and wait for atleast two consequtive frame
flags (stop+start).

Signed-off-by: Bjarki Arge Andreasen <bjarki@arge-andreasen.me>