mac80211: return if IBSS chandef can not be used

This was originally designed to fail when a 40+/40- mode can not be
used, but basic modes (such as 5/10/20 MHz) must be handled
with an error.

Signed-off-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>