Merge from RELENG_5 revision 1.51.2.3: fix the handling of the
UCS_RXSTOP flag so that it always tracks whether or not the receive
pipe is stopped. This ensures that we do not attempt to start the
same transfer twice, and it allows ucomstop() to skip the restarting
of the read pipe if it was not originally running, such as when
called indirectly from ucomreadcb().

Approved by: re (kensmith)