iio: iio_push_to_buffers(): Change type of 'data' to const void *

Change the type of the 'data' parameter for iio_push_to_buffers() from 'u8 *' to
'const void *'. Drivers typically use the correct type (e.g. __be16 *) for their
data buffer. When passing the buffer to iio_push_to_buffers() it needs to be
cast to 'u8 *' for the compiler to not complain (and also having to add __force
if we want to keep sparse happy as well). Since the buffer implementation should
not care about the data layout (except the size of one sample) using a void
pointer is the correct thing to do. Also make it const as the buffer
implementations are not supposed to modify it.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

include/linux/iio/consumer.h

@@ -77,7 +77,7 @@ struct iio_cb_buffer;
  * fail.
  */
 struct iio_cb_buffer *iio_channel_get_all_cb(struct device *dev,
-					     int (*cb)(u8 *data,
+					     int (*cb)(const void *data,
 						       void *private),
 					     void *private);
 /**