mwifiex: add VHT MCS rate configuration support

During Tx rate configuration, newer firmware V15 expects bitmap
for VHT MCS rates as well.

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>