rt2x00: Clean up RFCSR1 programming in rt2800_config_channel_rf3xxx.

Setting of the individual fields of the RF register can be simplified.

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>