mwifiex: fix hang issue for USB chipsets

mwifiex: fix hang issue for USB chipsets

Bug 60815 - Interface hangs in mwifiex_usb
https://bugzilla.kernel.org/show_bug.cgi?id=60815

We have 4 bytes of interface header for packets delivered to SDIO
and PCIe, but not for USB interface.

In Tx AMSDU case, currently 4 bytes of garbage data is unnecessarily
appended for USB packets. This sometimes leads to a firmware hang,
because it may not interpret the data packet correctly.

Problem is fixed by removing this redundant headroom for USB.

Cc: <stable@vger.kernel.org> # 3.5+
Tested-by: Dmitry Khromov <icechrome@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>