mac80211: allow mode change if IBSS is not allowed

Changing mode on an interface is not allowed if IBSS is disabled for the
current channel.  That restriction should only apply when switching to
the ad-hoc mode, as it was prior to "cfg80211: handle SIOCGIWMODE/SIOCSIWMODE".

Signed-off-by: Pavel Roskin <proski@gnu.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>