serial: MPSC: Remove race between Rx stop & restart

serial: MPSC: Remove race between Rx stop & restart

The patch in commit ID f8f0fae38828430b0400aa2f1443308ca517cde6 stops (aborts)
the MPSC's receive engine just before restarting it.  Unfortunately, it
doesn't wait for the abort to complete before restarting it which creates a
race between the abort and the restart.  If the restart occurs first, the
in-progress abort stops it again and the rx engine remains stopped.

Instead, do the abort when the SDMA engine is being stopped.  Make sure to
wait for the abort to complete before continuing.

Signed-off-by: Carlos Sanchez <carlos.sanchez@gecoinc.com>
Signed-off-by: Mark A. Greer <mgreer@mvista.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>