msm: mhi_dev: Do not flush events to host if channel is stopped

Do not flush completion events to host when the channel is
stopped or closed as the flush function might use invalid
data for the channel. Also add a check for channel state
in the write API.

Change-Id: I0e882fab4891cac60ddbb17f4be1dfeb502ecf92
Signed-off-by: Siva Kumar Akkireddi <sivaa@codeaurora.org>