iio:ad7266: Fix broken regulator error handling

All regulator_get() variants return either a pointer to a regulator or an
ERR_PTR() so testing for NULL makes no sense and may lead to bugs if we
use NULL as a valid regulator. Fix this by using IS_ERR() as expected.

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