mhi: core: Process bandwidth scaling event after force suspend

Bandwidth scaling event is handled by low priority worker. There
is a possibility of low priority worker racing with mission mode
state transition handling. This causes force suspend failure
because dev_wake count is incremented while processing bandwidth
scaling event. As a result l1ss timeout remains disabled and prevents
runtime suspend. Fix this issue by adding a flag to indicate force
suspend completion. This prevents processing of bandwidth scaling
event until force suspend is done. Bandwidth scaling events get
re-played after force suspend is done.

Change-Id: I0f2c415155842cab0449728dd136093be91f7259
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>