mwifiex: update MCS set as per RX-STBC bit from hostapd

To set AP in 1X1 or 2X2 mode through hostapd, we need to set
RX-STBC* in hostapd.conf file. Depending upon RX-STBC bit in
ht_cap IE received from hostapd, we need to update mcs set in
bss_cfg appropriately before starting AP.

Signed-off-by: Maithili Hinge <maithili@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>