rt2x00: Enable PA_PE bits in TX_PIN_CFG according to active band.

(split off from the earlier RT35xx patch submitted by Shiang)

There's no point in enabling the PA_PE bits for the bands that we are
not active on.

Signed-off-by: Shiang Tu <shiang_tu@ralinktech.com>
Signed-off-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>