mwifiex: use VHT MCS mask in set bitrate mask handler

As V15 firmware supports VHT rate configuration, we can use this
information received in set bitrate mask handler.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>