tty: n_null: remove optional ldops

Only tty_ldisc_ops::read() and ::write() of n_null behave differently than
the default ldops implementations. They return %EOPNOTSUPP instead of
%EIO. So keep only those two and remove the rest ldops as they are
superfluous.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20230810091510.13006-5-jirislaby@kernel.org


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