mwifiex: add support to configure VHT for AP mode

Currently, default VHT configuration from the firmware is used
for the VHT operations. Adding vhtcfg command to configure the
firmware based on input received from cfg.

Enable VHT for AP mode only when cfg80211_ap_settings has a
VHT IE i.e., when ieee80211ac is set to 1 in the hostapd.conf.

Signed-off-by: Yogesh Ashok Powar <yogeshp@marvell.com>
Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>