ntty_chars_in_buffer is overly complicated. Simplify it by inverting the
condition.

Side FIXME: how can dc or port be NULL here?
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210505091928.22010-29-jslaby@suse.czSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>