rt2x00: Block all unsupported modes

It was possible for unsupported operating modes
to be accepted by the add_interface callback function.
This patch will block those modes until proper support
has been implemented for them.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>