mwifiex: fix wrong 11ac bits setting in fw_cap_info

bit 14 is actually reserved and bit 12 & 13 should be used for
11ac capability in fw_cap_info.

Cc: <stable@vger.kernel.org> # 3.9+
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>