[SERIAL] Fix another SMP deadlock with modem status signal changes

[SERIAL] Fix another SMP deadlock with modem status signal changes
The original fix sent to Ingo for stop_tx didn't take account that
the start_tx and stop_tx methods can be called from the device
specific code under the port spinlock.  Consequently, we move the
spinlock to the callers of these methods.  Documentation updated
to reflect the change.