msm: ipa: flush the desc before feeding

Flush the completed descriptors every time
before feeding new descriptors to IPA BAM

Change-Id: Ibd7c0ccab0c3b9bd0907e4488b9287b8026fc8c8
Acked-by: Sunil Paidimarri <hisunil@qti.qualcomm.com>
Signed-off-by: Skylar Chang <chiaweic@codeaurora.org>