regulator: core: don't print an error when no regulator is found

Only print an error when _regulator_get() is expected to return a valid
regulator, that is when _regulator_get() is called from regulator_get() and
we're not using the dummy because we don't have full-constraints, or when
_regulator_get() is called from regulator_get_exclusive() in which case
returning a dummy is not allowed.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mark Brown <broonie@linaro.org>