tty: make everyone's write_room return >= 0

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

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210302062214.29627-44-jslaby@suse.cz


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