mac80211: only create default STA interface if supported

Drivers will support this, obviously, but this forces them to
set it up properly.

(This includes the fix posted as "mac80211: fix ifmodes check" and
tested in wireless-testing by Hin-Tak and others. -- JWL)

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Reported-by: Larry Finger <Larry.Finger@lwfinger.net>
Tested-by: Hin-Tak Leung <htl10@users.sourceforge.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>