carl9170: rx: mark expected switch fall-through

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

Addresses-Coverity-ID: 1056534 ("Missing break in switch")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/ath/carl9170/rx.c

@@ -766,6 +766,7 @@ static void carl9170_rx_untie_data(struct ar9170 *ar, u8 *buf, int len)
 
 			goto drop;
 		}
+		/* fall through */
 
 	case AR9170_RX_STATUS_MPDU_MIDDLE:
 		/*  These are just data + mac status */