tty: audit: unify to u8

Somewhere, we use 'char', somewhere 'unsigned char'. Unify to 'u8' as
the rest of the tty layer does.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20230810091510.13006-31-jirislaby@kernel.org


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