staging: iio: adc: ad7192: removed spi_device_id

This patch removes spi_device_id table and moves the
init data (id of the chip) in the .data field
of of_device_id table.

Signed-off-by: Alexandru Tachici <alexandru.tachici@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>