iio:buffer: Introduced a function to assign the buffer specific attrs.

This is a necessary step in taking the buffer implementation
opaque.

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

include/linux/iio/buffer.h

@@ -17,6 +17,8 @@
 
 struct iio_buffer;
 
+void iio_buffer_set_attrs(struct iio_buffer *buffer,
+			 const struct attribute **attrs);
 /**
  * INDIO_BUFFER_FLAG_FIXED_WATERMARK - Watermark level of the buffer can not be
  *   configured. It has a fixed value which will be buffer specific.