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: NGabor Juhos <juhosg@openwrt.org>
Acked-by: NStanislaw Gruszka <stf_xl@wp.pl>
Acked-by: NGertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>