mwifiex: add missing endian conversion for fw_tsf

commit 9795229752c31da0c5f8a7dc4c827665327b52f9 upstream.

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

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>