msm: ecm_ipa: clear outstanding packets upon pipe disconnect

This change clears the outstanding packets upon disconnect.
It is needed because after pipe disconnect no WRITE-DONE
events shall be sent from IPA driver so the netdev will
wait for packets that will never be acked.

Change-Id: Ic37afb10d371f4b1f9b2ad40b19f0e5992dafe86
Signed-off-by: Talel Shenhar <tatias@codeaurora.org>