iio: adc: mcp320x: Drop unnecessary of_device_id attributes

The driver sets a .data pointer for each .compatible string but never
calls of_device_get_match_data().  Instead, ADC properties are looked up
with spi_get_device_id().  The .data pointer is therefore unnecessary,
so drop it.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>