dfu: add free_entity() to struct dfu_entity

This allows the backend to free any resources allocated during the
relevant dfu_fill_entity_*() call. This will soon be used by the
SF backend.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
diff --git a/include/dfu.h b/include/dfu.h
index d5562dc..43814b3 100644
--- a/include/dfu.h
+++ b/include/dfu.h
@@ -110,6 +110,8 @@
 	int (*flush_medium)(struct dfu_entity *dfu);
 	unsigned int (*poll_timeout)(struct dfu_entity *dfu);
 
+	void (*free_entity)(struct dfu_entity *dfu);
+
 	struct list_head list;
 
 	/* on the fly state */