msm: ipa: set the mac header to 0 for rx skbs

packet_rcv_spkt is expecting to strip out the skb_mac_header.
Part of the alloc_skb, the mac header is setup ~0 which is causing
the skb data point to wrong location.

Change-Id: Ib8c14c029145da6a779ead21fb851b26677f4d0f
Signed-off-by: Bojun Pan <bojunp@codeaurora.org>