staging:iio: chan_spec - take extend_name constant.

I can't envision a case where this is not constant and we
don't seem to have any in tree, so lets clear up this loose
end.
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/iio/iio.h

@@ -164,7 +164,7 @@ struct iio_chan_spec {
 	long			info_mask;
 	long			event_mask;
 	const struct iio_chan_spec_ext_info *ext_info;
-	char			*extend_name;
+	const char		*extend_name;
 	const char		*datasheet_name;
 	unsigned		processed_val:1;
 	unsigned		modified:1;