iio: adc: xilinx: Fix "vccaux" channel .address

For the "vccaux" channel, read the VCCAUX register, not VCCINT.

Signed-off-by: Thomas Betker <thomas.betker@rohde-schwarz.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>