rt2x00: rt2800: use tx_power2 in rt2800_config_channel_rf3xxx

Get closer to what the ralink driver does by setting the rf register 13
to tx_power2 during channel switch.

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