[media] go7007-v4l2: warning fix: index is unsigned, so it will never be below 0

drivers/staging/media/go7007/go7007-v4l2.c: In function 'vidioc_qbuf':
drivers/staging/media/go7007/go7007-v4l2.c:815:2: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]

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