msm: mhi_dev: Fix trigger callback sequence

MHI device receives an interrupt once a start command is
to be processed and holds a lock. Since this lock is already
held remove holding the lock in the trigger callback as this
can lead to a deadlock.

Change-Id: Ib02273c1844a453498331be5c6021d1e912ee5da
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>