carl9170: Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a warning
by explicitly adding a break statement instead of letting the code fall
through to the next case.

Link: https://github.com/KSPP/linux/issues/115Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/04257418814755f081fa0ac14a61b01328cdc4ed.1605896059.git.gustavoars@kernel.org

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

@@ -840,6 +840,7 @@ static bool carl9170_tx_rts_check(struct ar9170 *ar,
 	case CARL9170_ERP_RTS:
 		if (likely(!multi))
 			return true;
+		break;
 
 	default:
 		break;