gst/rtp/gstrtpmp4vpay.c: Don't waste time looking for a config string if we have codec_info on the incomming caps.

Original commit message from CVS:
* gst/rtp/gstrtpmp4vpay.c: (gst_rtp_mp4v_pay_class_init),
(gst_rtp_mp4v_pay_setcaps):
Don't waste time looking for a config string if we have codec_info
on the incomming caps.