regulator: core: Fix the init of DT defined fixed regulators

When a regulator is defined using DT and it has a single voltage the
regulator init always tries to apply this voltage. However it fails if
the regulator isn't settable because it is using an internal low level
function. To overcome this we now first query the regulator and only
set it if needed.

Signed-off-by: Alban Bedel <alban.bedel@avionic-design.de>
Signed-off-by: Mark Brown <broonie@linaro.org>