iio: adc: ad7887: invert/rework external ref logic

This change inverts/reworks the logic to use an external reference via a
provided regulator.

Now the driver tries to obtain a regulator. If one is found, then it is
used. The rest of the driver logic already checks if there is a non-NULL
reference to a regulator, so it should be fine.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20201002082723.184810-1-alexandru.ardelean@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>