staging:iio:accel:adis16204 unwind use of IIO_CHAN macro.

This macro is being removed to simplify ongoing maintenance
so we need to unwind and remaining users.

Note that previously the supply was not indexed. I have made
it indexed for consistency with other similar devices and
for internal consistency with the aux adc port.

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