iio: adc: xilinx: check return value of xadc_write_adc_reg

In function xadc_probe, xadc_write_adc_reg can return an error value
when write fails. The fix checks for the return value consistent with
other invocations of the latter function.
Signed-off-by: NAditya Pakki <pakki001@umn.edu>
Reviewed-by: NMichal Simek <michal.simek@xilinx.com>
Signed-off-by: NJonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/xilinx-xadc-core.c

@@ -1273,8 +1273,10 @@ static int xadc_probe(struct platform_device *pdev)
 			xadc->threshold[i] = 0xffff;
 		else
 			xadc->threshold[i] = 0;
-		xadc_write_adc_reg(xadc, XADC_REG_THRESHOLD(i),
+		ret = xadc_write_adc_reg(xadc, XADC_REG_THRESHOLD(i),
 			xadc->threshold[i]);
+		if (ret)
+			goto err_free_irq;
 	}
 
 	/* Go to non-buffered mode */