tty: n_gsm: Mark expected switch fall-throughs

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

This patch fixes the following warnings:

drivers/tty/n_gsm.c: In function ‘gsm_dlci_data’:
drivers/tty/n_gsm.c:1582:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
if (tty) {
^
drivers/tty/n_gsm.c:1587:2: note: here
case 1:
^~~~
drivers/tty/n_gsm.c: In function ‘gsm1_receive’:
CC [M] drivers/scsi/snic/snic_disc.o
CC [M] drivers/net/wireless/realtek/rtlwifi/pci.o
CC drivers/usb/early/xhci-dbc.o
drivers/tty/n_gsm.c:1981:12: warning: this statement may fall through [-Wimplicit-fallthrough=]
gsm->fcs = INIT_FCS;
^
drivers/tty/n_gsm.c:1983:2: note: here
case GSM_ADDRESS: /* Address continuation */
^~~~

Warning level 3 was used: -Wimplicit-fallthrough=3

Notice that, in this particular case, the code comment is modified
in accordance with what GCC is expecting to find.

This patch is part of the ongoing efforts to enable
-Wimplicit-fallthrough.

Signed-off-by: Gustavo A. R. Silva <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/n_gsm.c

···
 		pr_debug("%d bytes for tty\n", len);
 	switch (dlci->adaption)  {
 	/* Unsupported types */
-	/* Packetised interruptible data */
-	case 4:
+	case 4:		/* Packetised interruptible data */
 		break;
-	/* Packetised uininterruptible voice/data */
-	case 3:
+	case 3:		/* Packetised uininterruptible voice/data */
 		break;
-	/* Asynchronous serial with line state in each frame */
-	case 2:
+	case 2:		/* Asynchronous serial with line state in each frame */
 		while (gsm_read_ea(&modem, *data++) == 0) {
 			len--;
 			if (len == 0)
···
 			gsm_process_modem(tty, dlci, modem, clen);
 			tty_kref_put(tty);
 		}
-	/* Line state will go via DLCI 0 controls only */
-	case 1:
+		/* Fall through */
+	case 1:		/* Line state will go via DLCI 0 controls only */
 	default:
 		tty_insert_flip_string(port, data, len);
 		tty_flip_buffer_push(port);
···
 		gsm->address = 0;
 		gsm->state = GSM_ADDRESS;
 		gsm->fcs = INIT_FCS;
-		/* Drop through */
+		/* Fall through */
 	case GSM_ADDRESS:	/* Address continuation */
 		gsm->fcs = gsm_fcs_add(gsm->fcs, c);
 		if (gsm_read_ea(&gsm->address, c))