regulator: core: Use list_voltage() to read single voltage regulators

If the regulator doesn't supply a way of reading back the voltage but does
provide a list_voltage() operation then use that with a selector of zero
to read the voltage.  Regulators doing this means that we have the list
operation there for consumers that want to configure themselves.

Reported-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>