msm: mhi: Prevent flush queue if channel is closed

Change is to prevent flush events being queued post channel close and
wait for pending flush event callbacks prior to channel close.

Change-Id: Ic168be0a187b3edcfeccedc565fb02673eaee883
Signed-off-by: Subramanian Ananthanarayanan <skananth@codeaurora.org>
Signed-off-by: Gauri Joshi <gaurjosh@codeaurora.org>