rt2x00: Enable support for RT53xx PCI devices by default.

Code seems to be feature-complete, so no reason to not enable
these devices by default.

Also, remove the sentence about the support for these devices being
non-functional.

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