staging: iio: cdc: ad7746: Use read_avail() rather than opencoding.

Switch over to the IIO core handling for _available attributes
making them available for in kernel users and enforcing correct
naming etc automatically.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220626122938.582107-14-jic23@kernel.org