regulator: core: Fix default return value for _get()

Now that we are defaulting to providing dummy regulators fix the logic
for substituting a dummy by making the default return code -EPROBE_DEFER.

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