mhi: core: block low power modes only in mission mode

While attempting to block low power modes before sending
commands to modify channel state, make sure that execution
environment is in mission mode as we may run into a race
condition where a forced suspend/resume cycle does not get
a chance to run.

Change-Id: Ibc7ca5a52036e18677bb597dc0cd95be96d6698a
Signed-off-by: Bhaumik Bhatt <bbhatt@codeaurora.org>