msm: mhi_dev: Use req_lock spinlock during client release

The change is to use req_lock spinlock during client release
and to use the same spinlock while checking for is_stale
during read/write completion callbacks.

Change-Id: I7ebafecb7503fa8521fa8f849cabf4b82bbc2f53
Signed-off-by: Subramanian Ananthanarayanan <skananth@codeaurora.org>
Signed-off-by: Gauri Joshi <gaurjosh@codeaurora.org>