mhi: core: handle low priority events after controller is ready

Controller gets a callback at mission mode entrypoint. Ensure
that is done before handling any bandwidth scaling requests
from low priority worker thread as there is a possibility of a
race.

Change-Id: I9592699a395021fd6f72a2b3e42ed8b80ec097aa
Signed-off-by: Bhaumik Bhatt <bbhatt@codeaurora.org>