iio: adc: ad7291: convert probe to device-managed only

iio: adc: ad7291: convert probe to device-managed only

This is a simple conversion for to device-managed with using
devm_request_threaded_irq(), disabling the regulator via a
devm_add_action_or_reset() hook and finally using
devm_iio_device_register().

The i2c_set_clientdata() call is removed as it becomes redundant after this
change.

Signed-off-by: Alexandru Ardelean <aardelean@deviqon.com>
Link: https://lore.kernel.org/r/20210926194315.7742-1-aardelean@deviqon.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>