rt2800: use eeprom OFDM 6M TX power as criterion

Don use TX_PWR_CFG_0 register value of OFDM 6M tx power as criterion
since it can be changed. The same do vendor driver (see
AsicAdjustSingleSkuTxPower and AsicGetTxPowerOffset functions from
2011_0719_RT3070_RT3370_RT5370_RT5372_Linux_STA_V2.5.0.3_DPO).

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>