mac80211: remove BSS from cfg80211 list when leaving IBSS

Remove BSS from cfg80211 BSS list if we are only member in IBSS when
leaving it.

Signed-off-by: Teemu Paasikivi <ext-teemu.3.paasikivi@nokia.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>