msm: mhi_dev: Update channel state on start command

After receiving channel start command update channel
state. This prevents channel read to fail after a
channel reset followed by start command.

Change-Id: Id534a4c65e8f5bba02fff814c1d763635ff9fb7e
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>