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.