iio: adc: intel_mrfld_adc: convert probe to full device-managed

iio: adc: intel_mrfld_adc: convert probe to full device-managed

The only call in the remove hook is the iio_map_array_unregister() call.
Since we have a devm_iio_map_array_register() function now, we can use that
and remove the remove hook entirely.
The IIO device was registered with the devm_iio_device_register() prior to
this change.

Also, the platform_set_drvdata() can be removed now, since it was used only
in the remove hook.

Signed-off-by: Alexandru Ardelean <aardelean@deviqon.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20210903072917.45769-3-aardelean@deviqon.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>