rt2x00: Fix tx parameter initialization

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: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>