[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)
^