mhi: core: handle disable transitions in EE state worker

It is possible that device asserts or is powered off while
handling mission mode transition. Synchronize device shutdown
or assert handling with processing of other execution
environments to avoid race conditions.

Change-Id: I5253de16b0b6438013b0588d835e65b7a1eea307
Signed-off-by: Bhaumik Bhatt <bbhatt@codeaurora.org>