serial: fix test of unsigned

The variables were unsigned so the tests did not work.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>