staging:iio:buffer add a cache of the timestamp scan index.

Basically avoids looking it up lots of times.
Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/staging/iio/buffer.h

@@ -106,6 +106,7 @@ struct iio_buffer {
 	int					scan_count;
 	long					*scan_mask;
 	bool					scan_timestamp;
+	unsigned				scan_index_timestamp;
 	const struct iio_buffer_access_funcs	*access;
 	const struct iio_buffer_setup_ops		*setup_ops;
 	struct list_head			scan_el_dev_attr_list;

drivers/staging/iio/industrialio-buffer.c

@@ -313,6 +313,9 @@ int iio_buffer_register(struct iio_dev *indio_dev,
 			if (ret < 0)
 				goto error_cleanup_dynamic;
 			attrcount += ret;
+			if (channels[i].type == IIO_TIMESTAMP)
+				buffer->scan_index_timestamp =
+					channels[i].scan_index;
 		}
 		if (indio_dev->masklength && buffer->scan_mask == NULL) {
 			buffer->scan_mask = kcalloc(BITS_TO_LONGS(indio_dev->masklength),