rt2x00: Add/Modify protection related register definitions

Make the definition of protection related registers more precisely

Signed-off-by: Shiang Tu <shiang_tu@ralinktech.com>
Acked-by: Helmut Schaa <helmut.schaa@googlemail.com>
Acked-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>