tty/n_hdlc: mark expected switch fall-through

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

This patch fixes the following warning:

drivers/tty/n_hdlc.c: In function ‘n_hdlc_tty_ioctl’:
drivers/tty/n_hdlc.c:775:3: warning: this statement may fall through [-Wimplicit-fallthrough=]
   switch (arg) {
   ^~~~~~
drivers/tty/n_hdlc.c:782:2: note: here
  default:
  ^~~~~~~

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 <gustavo@embeddedor.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/n_hdlc.c

@@ -777,7 +777,7 @@ static int n_hdlc_tty_ioctl(struct tty_struct *tty, struct file *file,
 		case TCOFLUSH:
 			flush_tx_queue(tty);
 		}
-		/* fall through to default */
+		/* fall through - to default */
 
 	default:
 		error = n_tty_ioctl_helper(tty, file, cmd, arg);