msm: mhi_dev: Add event lock

There can be multiple MHI channels sending completion event
after processing read/write operations. Add a synchronous
lock when sending event completion to the host to prevent
potential race condition.

Change-Id: Ic927437b7559a50f981e40676889cb33508c497e
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>