iio:adc: ad7766: testing the wrong variable in probe

We should be testing "ret" here.

Fixes: aa16c6bd ("iio:adc: Add support for AD7766/AD7767")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>