mwifiex: update 11n status as per start_ap IE

If HT IE is present in beacon_data of start_ap cfg80211 handler,
enable 11n flag in AP. Disable 11n flag when stop_ap handler is
called.

Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: Kiran Divekar <dkiran@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>