staging: speakup: signedness bug in spk_ttyio_in_nowait()

On most of the common arches char is signed so it can't ever == 0xff.
Let's fix this by making it a u8.

Fixes: 6b9ad1c7 ("staging: speakup: add send_xchar, tiocmset and input functionality for tty")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>