tty: xtensa/iss: drop unneeded tty_operations hooks

tty: xtensa/iss: drop unneeded tty_operations hooks

All ::flush_chars(), ::hangup(), and ::wait_until_sent() from struct
tty_operations are optional. There is no need to provide them with
empty bodies.

tty_operations::put_char() needs not be provided if it is the same as
tty_operations::write(tty, &ch, 1).

So drop all of them.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Cc: Chris Zankel <chris@zankel.net>
Cc: Max Filippov <jcmvbkbc@gmail.com>
Acked-by: Max Filippov <jcmvbkbc@gmail.com>
Link: https://lore.kernel.org/r/20230810091510.13006-2-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>