tty: tty_port: rename 'disc' to 'ld'

Line discipline variables are named 'ld' all over the tty code. Rename
these in tty_port, so that it is easier to grep for the code (namely for
"ld->ops").

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20230810091510.13006-7-jirislaby@kernel.org


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