mac80211: replace some dead code by a warning

Given the (nested) switch statements, this code can't
be reached, so make it warn instead of manipulating
the carrier state which seems purposeful.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
diff --git a/net/mac80211/iface.c b/net/mac80211/iface.c
index 8c94195..63b6367 100644
--- a/net/mac80211/iface.c
+++ b/net/mac80211/iface.c
@@ -581,7 +581,8 @@
 		case NL80211_IFTYPE_P2P_DEVICE:
 			break;
 		default:
-			netif_carrier_on(dev);
+			/* not reached */
+			WARN_ON(1);
 		}
 
 		/*