[media] tw686x: make const structs static

Fix sparse warnings:

tw686x-video.c:148:29: warning: symbol 'memcpy_dma_ops' was not declared. Should it be static?
tw686x-video.c:195:29: warning: symbol 'contig_dma_ops' was not declared. Should it be static?
tw686x-video.c:361:29: warning: symbol 'sg_dma_ops' was not declared. Should it be static?

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>