You do realize you're losing some of the functions that make the U360 great by running it that way, right?
Anyway, to remove the mapping MAME places on it (which shouldn't be the case since you're not running it via USB):
in the mame.ini change:
-joystick_map auto
to:
-joystick_map 7778...4445
Also, make sure the magnet is sitting flat on the end of the stick. Sometimes when screwing the balltop on, the slot gets damaged and it makes the magnet sit askew.
If none of this clears it up, you can increase the deadzone in the map you're using.