media: au0828: Use umin macro

Simplifies the code.

Found by cocci:
drivers/media/usb/au0828/au0828-video.c:605:11-12: WARNING opportunity for min()

Link: https://lore.kernel.org/linux-media/20240429-fix-cocci-v3-16-3c4865f5a4b0@chromium.org


Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>