mhi: core: Use high priority workqueue for all MHI work

Boot-up and shutdown require faster scheduling of the PM state
worker. Use the high priority workqueue for processing any of
the PM state transitions to ensure faster boot-up or shutdown
handling on MHI host.

Change-Id: Ie311426344d9561d490aa2bf6616aa30b4333e9a
Signed-off-by: Bhaumik Bhatt <bbhatt@codeaurora.org>