rt2800: add write_with_rx_chain function

Based on:
AsicBBPWriteWithRxChain()

from:
DPO_RT5572_LinuxSTA_2.6.1.3_20121022/chips/rtmp_chip.c

Signed-off-by: Stanislaw Gruszka <stf_xl@wp.pl>
Tested-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/drivers/net/wireless/rt2x00/rt2800.h b/drivers/net/wireless/rt2x00/rt2800.h
index 25c94b5..6105243 100644
--- a/drivers/net/wireless/rt2x00/rt2800.h
+++ b/drivers/net/wireless/rt2x00/rt2800.h
@@ -1939,6 +1939,9 @@
 #define BBP4_BANDWIDTH			FIELD8(0x18)
 #define BBP4_MAC_IF_CTRL		FIELD8(0x40)
 
+/* BBP27 */
+#define BBP27_RX_CHAIN_SEL		FIELD8(0x60)
+
 /*
  * BBP 47: Bandwidth
  */