msm: mhi-dev: Flush ereqs from dev close to avoid OOS transfers

Currently there is a race condition where during the flushing of ereqs
the channel is closed from the client on the EP side, which is causing
the flush function to access null memory. To avoid that call the flush
from dev close which ensures all the pending ereqs are closed before
closing the channel and freeing the memory.

Change-Id: Id81d8cb8b326340d28f6cfe9d48dcd685a8038f9
Signed-off-by: Gauri Joshi <gaurjosh@codeaurora.org>