modem: cmux: Patch resync mechanism

The CMUX resync mechanism can get stuck between states
MODEM_CMUX_RECEIVE_STATE_RESYNC_0 and
MODEM_CMUX_RECEIVE_STATE_RESYNC_1 if the resync flags, which are
sent only once in state MODEM_CMUX_RECEIVE_STATE_SOF, are not
responded to, or the response is lost.

This patch ensures resync flags are sent from states
MODEM_CMUX_RECEIVE_STATE_SOF, MODEM_CMUX_RECEIVE_STATE_RESYNC_1
and MODEM_CMUX_RECEIVE_STATE_RESYNC_2 if its determined that the
resync flags are not being responded to.

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