nl80211: allow 5 and 10 MHz channels for IBSS

Whether the wiphy supports it or not is already checked, so what is left
is to enable these channel types.

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>