Ensure that when an invalid value in ret or value is found -EINVAL
is returned. A previous commit broke the way the return error is
being returned and instead caused the return code in ret to be
re-assigned rather than be returned.

Fixes: 5d9854ea ("iio: hid-sensor: Store restore poll and hysteresis on S3")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>