Properly handle Linux event device button numbers

Old code would, by coincidence, map 1->16 to 1->16, then 17->32 to
1->32, leaving 1 and 17 on the same button number.

We still map 33->56 to 1->24, creating some overlap. But at least that's
because StepMania will have run out of buttons.

THIS PATCH WILL CHANGE THE CODES OF JOYSTICK BUTTONS 17 AND UP.