staging: iio/ad7291: fix error code in ad7291_probe()

We should be returning a negative error code instead of success here.

This would have been detected by GCC, except that the "ret" variable was
initialized with a bogus value to disable GCC's uninitialized variable
warnings.  I've cleaned that up, as well.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Cc: Stable@vger.kernel.org