rt2x00: move experimental on kconfig only to rt2800usb

These drivers have been around for a while, if there are issues
they should be reported. rt2800usb is still a bit flaky though.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>