[media] saa7134,saa7164: warning: comparison of unsigned fixes

drivers/media/pci/saa7134/saa7134-core.c:947:2: warning: comparison of unsigned expression >= 0 is always true [-Wtype-limits]
drivers/media/pci/saa7164/saa7164-core.c:413:3: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]
drivers/media/pci/saa7164/saa7164-core.c:489:3: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]
drivers/media/pci/saa7134/saa7134-video.c:2514:2: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>