mac80211/rc80211: add chandef to rate initialization

5 and 10 MHz support needs to know the current operating channel width,
add the chandef to the rate control API.

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>