tty: make everyone's chars_in_buffer return >= 0

The tty line disciplines don't expect tty_operations::chars_in_buffer to
return negative values. Fix the two drivers which violate this.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210302062214.29627-43-jslaby@suse.cz


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