msm: mhi_dev: Avoid race in event ring updates

Event ring can be shared by multiple channels. Synchronize
completion event updates to the event ring to avoid race
condition when the same ring is updated in the read and
write path.

Change-Id: Ib626bb45206766f34e7ae5224148b8dea962fc11
Signed-off-by: Siva Kumar Akkireddi <sivaa@codeaurora.org>