rt2800: radio 3xxx: reprogram only lower bits of RF_R3

Synchronize code with Ralink driver:
2011_0719_RT3070_RT3370_RT5370_RT5372_Linux_STA_V2.5.0.3_DPO
(functions: RT33xx_ChipSwitchChannel() and RT30xx_ChipSwitchChannel())
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/rt2x00/rt2800.h

@@ -1795,6 +1795,11 @@ struct mac_iveiv_entry {
  */
 #define RFCSR2_RESCAL_EN		FIELD8(0x80)
 
+/*
+ * RFCSR 3:
+ */
+#define RFCSR3_K			FIELD8(0x0f)
+
 /*
  * FRCSR 5:
  */

drivers/net/wireless/rt2x00/rt2800lib.c

@@ -1648,7 +1648,10 @@ static void rt2800_config_channel_rf3xxx(struct rt2x00_dev *rt2x00dev,
 	u8 rfcsr;
 
 	rt2800_rfcsr_write(rt2x00dev, 2, rf->rf1);
-	rt2800_rfcsr_write(rt2x00dev, 3, rf->rf3);
+
+	rt2800_rfcsr_read(rt2x00dev, 3, &rfcsr);
+	rt2x00_set_field8(&rfcsr, RFCSR3_K, rf->rf3);
+	rt2800_rfcsr_write(rt2x00dev, 3, rfcsr);
 
 	rt2800_rfcsr_read(rt2x00dev, 6, &rfcsr);
 	rt2x00_set_field8(&rfcsr, RFCSR6_R1, rf->rf2);