staging:iio:cdc:ad7150: Drop platform data support

There are no mainline board files using this driver so lets drop
the platform_data support in favour of devicetree and similar.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20210314181511.531414-6-jic23@kernel.org