v4l2: do not force NTSC as standard

Setting a standard is meaningful only for analog capture devices.

libavdevice/v4l2.c

@@ -730,7 +730,7 @@ static int v4l2_read_close(AVFormatContext *s1)
 #define OFFSET(x) offsetof(struct video_data, x)
 #define DEC AV_OPT_FLAG_DECODING_PARAM
 static const AVOption options[] = {
-    { "standard", "", offsetof(struct video_data, standard), FF_OPT_TYPE_STRING, {.str = "NTSC" }, 0, 0, AV_OPT_FLAG_DECODING_PARAM },
+    { "standard", "", offsetof(struct video_data, standard), FF_OPT_TYPE_STRING, {.str = NULL }, 0, 0, AV_OPT_FLAG_DECODING_PARAM },
     { "channel",  "", offsetof(struct video_data, channel),  FF_OPT_TYPE_INT,    {.dbl = 0 }, 0, INT_MAX, AV_OPT_FLAG_DECODING_PARAM },
     { "video_size", "A string describing frame size, such as 640x480 or hd720.", OFFSET(video_size), FF_OPT_TYPE_STRING, {.str = NULL}, 0, 0, DEC },
     { "pixel_format", "", OFFSET(pixel_format), FF_OPT_TYPE_STRING, {.str = NULL}, 0, 0, DEC },