staging: iio: ad7606: replace range/range_available with corresponding scale

Eliminate the non-standard attributes in_voltage_range and
in_voltage_range_available. Implement in_voltage_scale_available in place
of these attributes and update the SCALE accordingly. The array
scale_avail is introduced to hold the available scale values.

Signed-off-by: Eva Rachel Retuya <eraretuya@gmail.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>