iio: adc: ad7887: convert probe to device-managed functions

iio: adc: ad7887: convert probe to device-managed functions

This is conversion of the driver to use device-managed functions.
The IIO registration and triggered buffer setup both have device-managed
versions.
The regulator disable needs to be handled via an action_or_reset handler.

With these changes, the remove hook is removed, and the error path is
cleaned up in probe.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20201113091648.148589-1-alexandru.ardelean@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>