This has been an issue since I started mame (0.37u13). For a while there was a cool fix, but I'm not sure if it's still working (related stuff added at same timehave fallen be the way side over the year or two since they were added). I'm at
work, er "outside", so you'll have to test it, though.

Mame can use the full analog axis or half the axis. Most inputs use the full axis (analog joystick X axis for example). A few inputs are like yours, and use only half an analog axis; it could be either hardware (like your case) or software (rom or pc game). The way you used to do it was tab, input, scroll to the analog port (not Inc/Dec), enter, move axis you want to use, wait until mame registers the axis, enter move axis in the half you want to use. Very much like mapping a "not" in digital inputs, but analog does do nots, it switches between full and half axis instead.
If that doesn't work (anymore), let us know and I can help you edit the cfg file by hand.
