[SERIAL] Locking fixup (part 1)

[SERIAL] Locking fixup (part 1)
After the last few days of debugging, we've ended up with the caller
of the start_tx and stop_tx methods taking the per-port lock.  This
cset and the accompanying csets make the same change to some of the
other methods for consistency reasons.  Since these methods don't
contain a lot of code, it is better that they have consistent locking
rules.

This cset fixes up the stop_rx method.