iommu: msm: Fix a bug in mutex unlock

mutex_lock is being called instead of mutex_unlock.
In error handling, this can cause hangs as same lock
is being acquired recursively.

Change-Id: I698dd7e00c6ae79a2bf1b9392d6c894cbfbf8ed4
Signed-off-by: Vinay Kalia <vkalia@codeaurora.org>