ccid: Add comments clarifying msgb ownership on Rx/Tx */

Change-Id: I99210b16320a1bf9d54a381b023fe105e2dc8114
diff --git a/ccid/ccid_device.h b/ccid/ccid_device.h
index bc69523..d0fcdc0 100644
--- a/ccid/ccid_device.h
+++ b/ccid/ccid_device.h
@@ -59,6 +59,9 @@
 
 /* CCID operations */
 struct ccid_ops {
+	/* msgb ownership in below functions is transferred, i.e. whoever
+	 * provides the callback function must make sure to msgb_free() them
+	 * once transmission on IN or INT EP has completed. */
 	int (*send_in)(struct ccid_instance *ci, struct msgb *msg);
 	int (*send_int)(struct ccid_instance *ci, struct msgb *msg);
 };