demux: asf: fix ms-dvr truncated frames (fix #14414)

Since the copy mpeg2 packetizer is no longer accepting
mpg2 fourcc, the -broken- ms-dvr files are not
packetized.

(cherry picked from commit f7ffd0132121652b85ab5fd49ad4a2423172b182)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/demux/asf/asf.c

@@ -1344,7 +1344,7 @@ static int DemuxInit( demux_t *p_demux )
             if( fmt.i_codec == VLC_FOURCC( 'D','V','R',' ') )
             {
                 /* DVR-MS special ASF */
-                fmt.i_codec = VLC_FOURCC( 'm','p','g','2' ) ;
+                fmt.i_codec = VLC_CODEC_MPGV;
                 fmt.b_packetized = false;
             }