rt2800: document RF_R03 register bits [7:4]

Taken from:
2011_0719_RT3070_RT3370_RT5370_RT5372_Linux_STA_V2.5.0.3_DPO
(based on function RT33xx_ChipSwitchChannel)
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/rt2x00/rt2800.h

@@ -1799,6 +1799,9 @@ struct mac_iveiv_entry {
  * RFCSR 3:
  */
 #define RFCSR3_K			FIELD8(0x0f)
+/* Bits [7-4] for RF3320 (RT3370/RT3390), on other chipsets reserved */
+#define RFCSR3_PA1_BIAS_CCK		FIELD8(0x70);
+#define RFCSR3_PA2_CASCODE_BIAS_CCKK	FIELD8(0x80);
 
 /*
  * FRCSR 5: