tty: core: the rest to u8

There are still last minor users in the tty core that still reference
characters by the 'char' type. Switch them to u8.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20231206073712.17776-6-jirislaby@kernel.org


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