rt2x00: Add register definition for busy time on secondary channel

Add the register definition CH_BUSY_STA_SEC for reading the busy time
on the secondary channel in HT40 mode. Also update the comments about
channel busy/idle time registers to express the used unit.
Signed-off-by: NHelmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: NIvo van Doorn <IvDoorn@gmail.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

drivers/net/wireless/rt2x00/rt2800.h

@@ -747,15 +747,20 @@
 #define INT_TIMER_EN_GP_TIMER		FIELD32(0x00000002)
 
 /*
- * CH_IDLE_STA: channel idle time
+ * CH_IDLE_STA: channel idle time (in us)
  */
 #define CH_IDLE_STA			0x1130
 
 /*
- * CH_BUSY_STA: channel busy time
+ * CH_BUSY_STA: channel busy time on primary channel (in us)
  */
 #define CH_BUSY_STA			0x1134
 
+/*
+ * CH_BUSY_STA_SEC: channel busy time on secondary channel in HT40 mode (in us)
+ */
+#define CH_BUSY_STA_SEC			0x1138
+
 /*
  * MAC_STATUS_CFG:
  * BBP_RF_BUSY: When set to 0, BBP and RF are stable.