tty: ldisc: document that ldops are optional

There is no need to provide any hook in struct tty_ldisc_ops. Document
that and write down that read/write return EIO in that case. The rest is
simply ignored.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20230810091510.13006-3-jirislaby@kernel.org


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