iio: adc: ad7793: use read_avail iio hook for scale available

This change uses the read_avail and '.info_mask_shared_by_type_available'
modifier to set the available scale.
Essentially, nothing changes to the driver's ABI.

The main idea for this patch is to remove the AD7793 driver from
checkpatch's radar. There have been about ~3 attempts to fix/break the
'in_voltage-voltage_scale_available' attribute, because checkpatch assumed
it to be an arithmetic operation and people were trying to change that.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>