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

In the Linux kernel, the following vulnerability has been resolved: 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.