iio: adc: xilinx-ams: fix return error variable

Return irq instead of ret which always equals to zero here.

Fixes: d5c70627

 ("iio: adc: Add Xilinx AMS driver")
Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Lv Ruyi <lv.ruyi@zte.com.cn>
Reviewed-by: Michal Simek <michal.simek@amd.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>