rt2x00: Replace open coded interface checking with interface combinations.

Mac80211 has formal infrastructure to specify which interface combinations
are supported. Make use of this facility in favor of open coding it
ourselves.
So far we only have to specify we can support multiple AP interfaces,
no other combinations are supported.

Inspired by an earlier patch from Paul Fertser.

Signed-off-by: Gertjan van Wingerde <gwingerde@gmail.com>
Cc: Paul Fertser <fercerpav@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>