mctp: serial: remove unnecessary ldisc data check

Jiri assures me that a ldisc->open with tty->disc_data set should never
happen, so this check doesn't do anything.

Reported-by: Jiri Slaby <jirislaby@kernel.org>
Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/mctp/mctp-serial.c

@@ -439,9 +439,6 @@ static int mctp_serial_open(struct tty_struct *tty)
 	if (!tty->ops->write)
 		return -EOPNOTSUPP;
 
-	if (tty->disc_data)
-		return -EEXIST;
-
 	idx = ida_alloc(&mctp_serial_ida, GFP_KERNEL);
 	if (idx < 0)
 		return idx;