tty: n_tty: remove unsigned char casts from character constants

We compile with -funsigned-char, so all character constants are already
unsigned chars. Therefore, remove superfluous casts.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20230827074147.2287-10-jirislaby@kernel.org


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