iio: adc: lp8788_adc: convert probe to full-device managed

iio: adc: lp8788_adc: convert probe to full-device managed

This change converts the probe of this driver to use device-managed
functions only, which means that the remove hook can be removed.
The remove hook has only 2 calls to iio_device_unregister() and
iio_map_array_unregister(). Both these can now be done via devm register
functions, now that there's also a devm_iio_map_array_register() function.

The platform_set_drvdata() can also be removed now.

This change also removes the error print for when the iio_device_register()
call fails. This isn't required now.

Signed-off-by: Alexandru Ardelean <aardelean@deviqon.com>
Link: https://lore.kernel.org/r/20210903072917.45769-5-aardelean@deviqon.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>