msm: ipa: wwan: release producer only when no outstanding packets

release WWAN_PROD only after getting WRITE_DONE
event for the last packet to make sure that there
is no outstanding packets when releasing WWAN_PROD.
This is fix for potential bug that A2 may go to
power collapse while there are packets pending in BAM

Change-Id: I6ae08fb149f4309866b693f37744e090581f5816
Signed-off-by: Skylar Chang <chiaweic@codeaurora.org>