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>