iio: xilinx-xadc: Fix ADC-B powerdown

The check for shutting down the second ADC is inverted. This causes it to
be powered down when it should be enabled. As a result channels that are
supposed to be handled by the second ADC return invalid conversion results.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Fixes: bdc8cda1 ("iio:adc: Add Xilinx XADC driver")
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>