regulator: core: Always use return value when regulator_dev_lookup() fails

Ensure that the return value is always set when we return now that the
logic has changed for regulator_get_optional() so we don't get missing
codes leaking out.

Reported-by: Thierry Reding <treding@nvidia.com>
Tested-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Mark Brown <broonie@linaro.org>