staging: iio: adc: ad7192: Remove platform data

This patch removes the reference voltage entry from the platform_data
structure. This is no longer needed since the reference voltage is obtained
from the device tree. With this we also remove the entire ad7192.h file.

The undefined reference voltage warning is promoted to an error signaling a
problem with the device tree.

Signed-off-by: Mircea Caprioru <mircea.caprioru@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>