rt2x00: Move interface type assignments to generic code.

Make sure all drivers can benefit of the assignment of the interface
type of an adapter, instead of keeping it for rt2800 only.

Signed-off-by: Gertjan van Wingerde <gwingerde@gmail.com>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Acked-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>