rtl8xxxu: Fix missing break in switch

Add missing break statement in order to prevent the code from falling
through to the default case.

Fixes: 26f1fad2 ("New driver: rtl8xxxu (mac80211)")
Signed-off-by: NGustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: NKalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/realtek/rtl8xxxu/rtl8xxxu_core.c

@@ -5692,6 +5692,7 @@ static int rtl8xxxu_set_key(struct ieee80211_hw *hw, enum set_key_cmd cmd,
 		break;
 	case WLAN_CIPHER_SUITE_TKIP:
 		key->flags |= IEEE80211_KEY_FLAG_GENERATE_MMIC;
+		break;
 	default:
 		return -EOPNOTSUPP;
 	}