[media] tw686x-kh: use the cached value

the dma_requests field is cached, but cache is not used:

drivers/staging/media/tw686x-kh/tw686x-kh-video.c: In function 'tw686x_video_irq':
drivers/staging/media/tw686x-kh/tw686x-kh-video.c:622:6: warning: variable 'requests' set but not used [-Wunused-but-set-variable]
  u32 requests;
      ^

Use the cache instead, as it seems reading it needs to be done
with spin lock taken.

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