staging: iio: adc: ad7192: Use read_avail for available attributes

This patch removes the in_voltage-voltage_scale_available and
in_voltage_scale_available attributes. These are replaced with available
attributes added in info_mask_shared_by_type_available.

With this modification the iio core will be responsible with naming the
attributes thus removing the checkpatch warning related to
in_voltage-voltage_scale_available naming.

Signed-off-by: Mircea Caprioru <mircea.caprioru@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>