iio: adc: men_z188_adc: Fix a resource leak in an error handling path

If iio_device_register() fails, a previous ioremap() is left unbalanced.

Update the error handling path and add the missing iounmap() call, as
already done in the remove function.

Fixes: 74aeac4da66f ("iio: adc: Add MEN 16z188 ADC driver")
Signed-off-by: Christophe JAILLET <[e-mail]>
Link: https://lore.kernel.org/r/320fc77786[e-mail]
Cc: <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/adc/men_z188_adc.c

···
 	struct z188_adc *adc;
 	struct iio_dev *indio_dev;
 	struct resource *mem;
+	int ret;
 
 	indio_dev = devm_iio_device_alloc(&dev->dev, sizeof(struct z188_adc));
 	if (!indio_dev)
···
 	adc->mem = mem;
 	mcb_set_drvdata(dev, indio_dev);
 
-	return iio_device_register(indio_dev);
+	ret = iio_device_register(indio_dev);
+	if (ret)
+		goto err_unmap;
 
+	return 0;
+
+err_unmap:
+	iounmap(adc->base);
 err:
 	mcb_release_mem(mem);
 	return -ENXIO;