V4L/DVB (7105): ivtv-yuv.c: make 3 functions static

This patch makes the following needlessly global functions static:
- ivtv_yuv_next_free()
- ivtv_yuv_setup_frame()
- ivtv_yuv_udma_frame()

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Reviewed-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>