[media] cx25821-video: declare cx25821_vidioc_s_std as static

Fixes the following warning:

	drivers/media/pci/cx25821/cx25821-video.c: At top level:
	drivers/media/pci/cx25821/cx25821-video.c:766:5: warning: no previous prototype for 'cx25821_vidioc_s_std' [-Wmissing-prototypes]

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