mac80211: Disallow concurrent IBSS/STA mode interfaces

Disallow having more than one IBSS interface up at any time
because of beacon distribution issues, and for now also disallow
having more than one IBSS/STA interface up at the same time
because we use the master interface's BSS struct which would
be completely corrupted when we have more than one up.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>