iio:ad7797: Use correct attribute_group

It should use ad7797_attribute_group in ad7797_info,
according to commit ("iio:ad7793: Add support for the ad7796 and ad7797").

Scale is fixed for the ad7796 and not programmable, hence
should not have the scale_available attribute.

Fixes: fd1a8b91

 ("iio:ad7793: Add support for the ad7796 and ad7797")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>