iio: adc: ti-ads7950: remove chip_info[]

Remove the chip_info[] array and related enum used for looking up chip-
specific information. Instead, use individual structs directly in the
module device tables.

Also update to use spi_get_device_match_data() in case the devicetree
table is ever used instead of the SPI device ID table.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>