rt2x00: Add debugfs access for rfcsr register

RFCSR is only used in rt2800. For other chipsets, the debug struct
for rfcsr should be zeroed, which isn't be an issue, since the code
can now cope with that.

Signed-off-by: Anisse Astier <anisse@astier.eu>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>