staging:iio:dac:max517: Convert to channel spec

Convert the max517 driver to channel spec. As part of the conversion the
"out_voltage_1&2_raw" property, which updates both channel 1 and 2
simultaneously with the same value, is lost, since this is not really covered by
the IIO spec and has only a limited use case in practice.

Also the channel index for the sysfs files is now zero based instead of one
based, which means all channel numbers will be lower by one. E.g.
"out_voltage_1_scale" instead of "out_voltage_2_scale"

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Acked-by: Roland Stigge <stigge@antcom.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>