msm: rndis_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: Ia565e81800fe6f397b608d80f6c824c312ecbf83
Signed-off-by: Talel Shenhar <tatias@codeaurora.org>