iio:ad7266: Fix probe deferral for vref

iio:ad7266: Fix probe deferral for vref

commit 68b356eb3d9f5e38910fb62e22a78e2a18d544ae upstream.

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>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>