iio: adc: xilinx-xadc: assign auxiliary channels address correctly

This patch fixes incorrect logic for assigning address
to auxiliary channels of xilinx xadc.

Signed-off-by: Subbaraya Sundeep Bhatta <sbhatta@xilinx.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Cc: Stable@vger.kernel.org
Signed-off-by: Jonathan Cameron <jic23@kernel.org>