mwifiex: add missing endian conversion for fw_tsf

It is u64 data received from firmware. Little endian to cpu
conversion is required here.

Cc: <stable@vger.kernel.org> # 3.5+
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>