msm: mhi_dev: allocate high priority Workqueue for mhi_sm_wq

Allocate high priority work queue for mhi_sm_wq such that
MHI state change events are handled with priority.

Change-Id: I3d95e23b3daafce5a8888e5203caa75f1744351b
Signed-off-by: Nitesh Gupta <nitegupt@codeaurora.org>