iio: iio_buffer_register: Skip channels with negative scan index

It is not always the case that all channels can be used in buffered mode. This
patch allows channels, which can not be used in buffered mode, to set their scan
index to a negative number, which will cause iio_buffer_register to ignore the
channel.
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/iio/industrialio-buffer.c

@@ -285,6 +285,9 @@ int iio_buffer_register(struct iio_dev *indio_dev,
 	if (channels) {
 		/* new magic */
 		for (i = 0; i < num_channels; i++) {
+			if (channels[i].scan_index < 0)
+				continue;
+
 			/* Establish necessary mask length */
 			if (channels[i].scan_index >
 			    (int)indio_dev->masklength - 1)