iio: adc: ad7768-1: replace manual attribute declaration

iio: adc: ad7768-1: replace manual attribute declaration

Use read_avail callback from struct iio_info to replace the manual
declaration of sampling_frequency_available attribute.

Reviewed-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Marcelo Schmitt <marcelo.schmitt@analog.com>
Signed-off-by: Jonathan Santos <Jonathan.Santos@analog.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://patch.msgid.link/b2653d270131b2c873373a6f81cde9a5bdf5d1ff.1749569957.git.Jonathan.Santos@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>