iio: adc: ti-ads8688: use read_avail for available attributes

Convert the in_voltage_scale_available and in_voltage_offset_available
attributes from legacy IIO_DEVICE_ATTR with custom show functions to the
IIO framework's read_avail callback. This uses the framework's built-in
support for _available attributes, removing the need for manual sysfs
formatting.

Precompute the available scale values at probe time since they depend on
the reference voltage which does not change after initialization.

Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Signed-off-by: Gabriel Rondon <grondon@gmail.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>