media: em28xx: use a default format if TRY_FMT fails

Follow the V4L2 spec, as warned by v4l2-compliance:

	warn: v4l2-test-formats.cpp(732): TRY_FMT cannot handle an invalid pixelformat.
	warn: v4l2-test-formats.cpp(733): This may or may not be a problem. For more information see:

warn: v4l2-test-formats.cpp(734): http://www.mail-archive.com/linux-media@vger.kernel.org/msg56550.html

Cc: stable@vger.kernel.org
Fixes: bddcf633 ("V4L/DVB (9927): em28xx: use a more standard way to specify video formats")
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

drivers/media/usb/em28xx/em28xx-video.c

@@ -1471,9 +1471,9 @@ static int vidioc_try_fmt_vid_cap(struct file *file, void *priv,
 
 	fmt = format_by_fourcc(f->fmt.pix.pixelformat);
 	if (!fmt) {
-		em28xx_videodbg("Fourcc format (%08x) invalid.\n",
-				f->fmt.pix.pixelformat);
-		return -EINVAL;
+		fmt = &format[0];
+		em28xx_videodbg("Fourcc format (%08x) invalid. Using default (%08x).\n",
+				f->fmt.pix.pixelformat, fmt->fourcc);
 	}
 
 	if (dev->board.is_em2800) {