rt2800: limit TX_PWR_CFG_ values to 0xc

Based on AsicAdjustTxPower function from vendor driver
(2011_0719_RT3070_RT3370_RT5370_RT5372_Linux_STA_V2.5.0.3_DPO)
limit per rate TX power values we program into TX_PWR_CFG_ registers.

Note that on some configurations (devices/rates) is allowed to use
bigger values than 0xc, but we use safe maximum value for now. Further
work need to be done to allow use bigger values than 0xc.

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>