rt2x00: Align rt2800 register initialization with vendor driver.

Align the rt2800 register initializations with the latest versions of the
Ralink vendor driver.
This patch is also preparation for the addition of support for RT3070 /
RT3071 / RT3090 / RT3390 based devices.

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