[media] ths7303: Declare as static a private function

git grep shows that the function is only called from ths7303.c
Fix this build warning:
CC      drivers/media/i2c/ths7303.o
drivers/media/i2c/ths7303.c:86:5: warning: no previous prototype for  ‘ths7303_setval’ [-Wmissing-prototypes]
   int ths7303_setval(struct v4l2_subdev *sd, enum ths7303_filter_mode mode)
        ^

Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>