ath5k: fix build break from "ath5k: Print out opmode in debugfs"

Also improve ath_opmode_to_string usage by having it return UNKNOWN
rather than NULL in the event of failure to map the opmode value to a
representative string.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/ath/debug.c

@@ -55,7 +55,7 @@ const char *ath_opmode_to_string(enum nl80211_iftype opmode)
 	case NL80211_IFTYPE_P2P_GO:
 		return "P2P-GO";
 	default:
-		return NULL;
+		return "UNKNOWN";
 	}
 }
 EXPORT_SYMBOL(ath_opmode_to_string);