iio:ad7266: Fix probe deferral for vref

Currently the ad7266 driver treats any failure to get vref as though the
regulator were not present but this means that if probe deferral is
triggered the driver will act as though the regulator were not present.
Instead only use the internal reference if we explicitly got -ENODEV which
is what is returned for absent regulators.

Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>