v4l2: clear cached caps on close

A different device with different caps may be used for the next open.

https://bugzilla.gnome.org/show_bug.cgi?id=712611

sys/v4l2/gstv4l2object.c

@@ -895,6 +895,8 @@ gst_v4l2_object_close (GstV4l2Object * v4l2object)
   if (!gst_v4l2_close (v4l2object))
     return FALSE;
 
+  gst_caps_replace (&v4l2object->probed_caps, NULL);
+
   if (v4l2object->formats) {
     gst_v4l2_object_clear_format_list (v4l2object);
   }