regulator: core: Handle full constraints systems when resolving supplies

When resolving device supplies if we fail to look up the regulator we
substitute in the dummy supply instead if the system has fully specified
constraints. When resolving supplies for regulators we do not have the
equivalent code and instead just directly use the regulator_dev_lookup()
result causing spurious failures.

This does not affect DT systems since we are able to detect missing
mappings directly as part of regulator_dev_lookup() and so have appropriate
handling in the DT specific code.

Reported-by: Christian Hartmann <cornogle@googlemail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>