mwifiex: fix warning while starting BSS

We see this warning while starting mwifiex AP:
Unsupported RX-STBC, default to 2x2

This was happening because of wrong offset while copying HT
capabilities from BSS configuration of start_ap handler.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>