mhi: core: Prevent suspend until channel operation is complete

Host allows to go to fast suspend while waiting for channel operations
such as start/stop to complete. Upon completion of channel ops host is
unable to respond back to remote clients in time because it is suspended.
Fix this issue by incrementing pending packet count as well as blocking
runtime/system suspend until channel operation is completed.

CRs-Fixed: 2528767
Change-Id: I9c2cff94cdc256ad529688ffbfd05883af9fb85f
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>