msm: mhi_dev: Added mutex lock in mhi_dev_write_channel

mhi_dev_write_channel is called by diag channel. While processing it,
reset interrupt is received from host. During the reset sequence the
work queue mhi_sm_wq is getting destroyed in mhi_dev_sm_exit API. When
the mhi_dev_write_channel is resumed, queuing of work is done as part of
mhi_dev_notify_sm_event. Here, as the work queue is destroyed, crash
occurred with a kernel null pointer deference error. This is a race
condition between reset sequence and mhi_dev_notify_sm_event.

To avoid this race condition added a mutex lock mhi_lock in
mhi_dev_write_channel before calling mhi_dev_notify_sm_event.

Change-Id: Idaf1c33c462b6d659f3e5ddb333afe9c6a967fac
Signed-off-by: Sai Chaitanya Kaveti <quic_skaveti@quicinc.com>