msm: ipa: add exception handler to teth_bridge

For RmNet and MBIM tethering there are some corner cases where USB
starts sending packets to IPA before modem setup the data call.
These packets arrive to AP as exception packets, and needs to be dropped.
This change adds exception packet handler to teth_bridge to drop these
packets and free the corresponding skb.

Change-Id: I388647aa83d40717b72a9854f465fde445f349f0
Acked-by: Ady Abraham <adya@qti.qualcomm.com>
Signed-off-by: Nadine Toledano <nadinet@codeaurora.org>