regulator: Use _regulator_get_voltage() consistently

Rather than referencing the get_voltage() operation directly in the
ops struct use the internal _regulator_get_voltage() API call to do
so, facilitating refactoring.

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