iio: adc: mcp320x: Set struct spi_driver .of_match_table

The driver has an OF id table but the .of_match_table is not set so
the SPI core can't do an OF style match and the table was unused.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Acked-by: Michael Welling <mwelling@ieee.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>