iio: adc: berlin2-adc: convert probe to device-managed only

This driver requires only a devm_add_action_or_reset() hook for the
power-down of the device, and then devm_iio_device_register() can be used
directly.

Signed-off-by: Alexandru Ardelean <aardelean@deviqon.com>
Link: https://lore.kernel.org/r/20210926192642.4051329-1-aardelean@deviqon.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>