mwifiex: populate rates in probe request using cfg80211_scan_request

Whenever available, use cfg80211_scan_request to populates rates
in outgoing probe request. This will help to advertise band specific
rates and fix an issue where 11b rates were advertised in probe
request going out on 11a band.

This will also ensure that we do not advertise 11b rates while P2P
scan is going on.

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