msm: mhi_dev: Ensure the flush list is empty for a stopped channel

If a channel gets stopped, while there are some outstanding requests/
transfers with IPA, then we simply ignore the completion from IPA.
But we must have already added the transfer-completion-event to the
flush list before submitting the transfer to IPA.

There is a possibility that we flush these stale events along with new
flush events (for new requests) when the channel gets re-started.
This un-intentional completion event can lead to out-of-sequence
events at the host.

So while stopping a channel, ensure all elements in the flush list
of that channel are discarded and the flush list is empty.

Change-Id: I04445b1f9f4d2d4fce0b8a96277517de14481ead
Signed-off-by: Veerabhadrarao Badiganti <vbadigan@codeaurora.org>