cfg80211/mac80211: get mandatory rates based on scan width

Mandatory rates for 5 and 10 MHz are different from the rates used for
20 MHz in 2.4 GHz mode, as they use OFDM only.

Signed-off-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
Signed-off-by: Mathias Kretschmer <mathias.kretschmer@fokus.fraunhofer.de>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>