media: aspeed-video: address a protential usage of an unitialized var

While this might not occur in practice, if the device is doing
the right thing, it would be teoretically be possible to have
both hsync_counter and vsync_counter negatives.

If this ever happen, ctrl will be undefined, but the driver
will still call:

	aspeed_video_update(video, VE_CTRL, 0, ctrl);

Change the code to prevent this to happen.

This was warned by cppcheck:

	[drivers/media/platform/aspeed-video.c:653]: (error) Uninitialized variable: ctrl

Reviewed-by: Eddie James <eajames@linux.ibm.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>