[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 <[e-mail]>
Acked-by: Laurent Pinchart <[e-mail]>
Acked-by: Lad, Prabhakar <[e-mail]>
Signed-off-by: Hans Verkuil <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

drivers/media/i2c/ths7303.c

···
 }
 
 /* following function is used to set ths7303 */
-int ths7303_setval(struct v4l2_subdev *sd, enum ths7303_filter_mode mode)
+static int ths7303_setval(struct v4l2_subdev *sd,
+			  enum ths7303_filter_mode mode)
 {
 	struct i2c_client *client = v4l2_get_subdevdata(sd);
 	struct ths7303_state *state = to_state(sd);