atm: firestream: Use fallthrough pseudo-keyword

Add pseudo-keyword macro fallthrough[1]

[1] https://www.kernel.org/doc/html/latest/process/deprecated.html?highlight=fallthrough#implicit-switch-case-fall-throughSigned-off-by: Wei Ming Chen <jj251510319013@gmail.com>
Link: https://lore.kernel.org/r/20210507123843.10602-1-jj251510319013@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/atm/firestream.c

@@ -795,6 +795,7 @@ static void process_incoming (struct fs_dev *dev, struct queue *q)
 		switch (STATUS_CODE (qe)) {
 		case 0x1:
 			/* Fall through for streaming mode */
+			fallthrough;
 		case 0x2:/* Packet received OK.... */
 			if (atm_vcc) {
 				skb = pe->skb;