Minor improvements to parseGenericController()

[kevinmoran]

• When calling HidP_GetUsageValue for axes/dpad, use NotRange.Usage instead of Range.UsageMin. This wasn't a bug because these are overlapping members in a typed union, but valueCaps[i].IsRange is false for these inputs so this is the intended usage, think it's better for clarity
• Replace hardcoded usage values with the defines for readability
• Fixed a probably trivial bug where buttonCount was always calculated from first element in buttonCaps array - For the gamepads I tested all the buttons are always in the buttonCaps[0] so I doubt this was ever hit, but it gave me some confusion in the debugger for my Dualsense controller which has a second buttonCap

Thanks!