mwifiex: add cfg80211 handler for set_bitrate_mask

Currently, setting only legacy bitrates on 2.4GHz band
are supported. Mode 802.11b/g/bg is enabled based on
bitrates selection. If only CCK bitrates selected then
802.11b mode is enabled. If only OFDM bitrates are
selected then 802.11g mode is enabled. For both: CCK
and OFDM rates 802.11bg mixed mode is enabled.

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