rt2x00: rt2800lib: add TX power configuration for RT3593

Based on the Ralink DPO_RT5572_LinuxSTA_2.6.0.1_20120629
driver.

References:
  RTMPReadTxPwrPerRateExt in chips/rt3593.c
  RT3593_AsicGetTxPowerOffset in chips/rt3593.c

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Acked-by: Stanislaw Gruszka <stf_xl@wp.pl>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>