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: 74aeac4d ("iio: adc: Add MEN 16z188 ADC driver")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/320fc777863880247c2aff4a9d1a54ba69abf080.1643445149.git.christophe.jaillet@wanadoo.fr
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>