mwifiex: parse rate info for AP

This patch adds support for parsing rates from cfg80211_ap_settings
of start_ap handler. This in turn adds support for 11a and 11b phy
configurations.

Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: Kiran Divekar <dkiran@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>