regulator: Fix WM831x regulator ID lookups for multiple WM831xs

With multiple wm831x devices the device IDs used for the regulators will
not always be contiguous so simply taking the modulus is not sufficient
to look up the ID, we need to reverse the way the ID is generated.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>