ath9k: Handle chainmask for A9280

The default chainmask for AR9280 is 3, so handle that when setting
the supported MCS rates. Also, check for the HW supported chainmask
when choosing single/dual stream.

Signed-off-by: Sujith <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/drivers/net/wireless/ath9k/main.c b/drivers/net/wireless/ath9k/main.c
index ede8980..04a6051 100644
--- a/drivers/net/wireless/ath9k/main.c
+++ b/drivers/net/wireless/ath9k/main.c
@@ -876,6 +876,7 @@
 	case 1:
 		ht_info->mcs.rx_mask[0] = 0xff;
 		break;
+	case 3:
 	case 5:
 	case 7:
 	default: