msm: dma: unify flush and stop_cmd interfaces

Previously there were two interfaces for flushing a channel, the
difference between them being rather ill defined. Both would end up
with the same result if used correctly, and they only varied in error
conditions.

Change-Id: If8dcf1d1b7fc2e7417508b67ca3ceeffe77fa658
Signed-off-by: Jeff Ohlstein <johlstei@codeaurora.org>