s390/3215: add switch fall through comment for -Wimplicit-fallthrough

Silence the following warning when built with -Wimplicit-fallthrough=3
enabled by default since 5.3-rc2:
drivers/s390/char/con3215.c: In function 'raw3215_irq':
drivers/s390/char/con3215.c:399:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
399 | if (dstat == 0x08)
| ^
drivers/s390/char/con3215.c:401:2: note: here
401 | case 0x04:
| ^~~~

Signed-off-by: Vasily Gorbik <[e-mail]>

drivers/s390/char/con3215.c

···
 		}
 		if (dstat == 0x08)
 			break;
+		/* else, fall through */
 	case 0x04:
 		/* Device end interrupt. */
 		if ((raw = req->info) == NULL)