mwifiex: fix wrong eth_hdr usage for bridged packets in AP mode

The eth_hdr is never defined in this driver but it gets compiled
without any warning/error because kernel has defined eth_hdr.

Fix it by defining our own p_ethhdr and use it instead of eth_hdr.

Cc: <stable@vger.kernel.org> # 3.7+
Signed-off-by: Ujjal Roy <royujjal@gmail.com>
Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>