local: Fix detection of scan-element channels

Previously, the detection of buffer-capable channels would fail when the
channel had attributes outside the scan_elements/ folder, as the channel
would be correctly marked as non-buffer-capable when the first attribute
was found, but was never changed to buffer-capable when scan-elements
attributes were later found.

It went unnoticed until now, most likely because there is generally more
than one channel of a given type (e.g. "voltage") in the scan elements,
and those channels of the same type share common attributes.

Fixes #740.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>

local.c

@@ -1379,6 +1379,8 @@ static int add_attr_to_channel(struct iio_channel *chn,
 		if (ret < 0)
 			goto err_free_fn;
 
+		chn->is_scan_element = true;
+
 		return 0;
 	}