rt2x00: Fix rt2800 txpower setting to correct value

TX_PWR_CFG_* setting need to consider below cases
-compesate 20M/40M tx power delta for 2.4/5GHZ band
-limit maximum EIRP tx power to power_level of
 regulatory requirement

Signed-off-by: RA-Jay Hung <jay_hung@ralinktech.com>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>