Check if the aifs, cw_min and cw_max are above 0
when determining if the default should be used.
Tor aifs a negative number is used to determine
if the default should be used or not.
Signed-off-by: NIvo van Doorn <IvDoorn@gmail.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>