staging: iio: ad7606: rework regulator handling

Currently, this driver ignores all errors from regulator_get(). The way
it is now, it also breaks probe deferral (EPROBE_DEFER). The correct
behavior is to propagate the error to the upper layers so they can
handle it accordingly.

Rework the regulator handling so that it matches the standard behavior.
If the specific design uses a static always-on regulator and does not
explicitly specify it, regulator_get() will return the dummy regulator.

Suggested-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Eva Rachel Retuya <eraretuya@gmail.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>