[media] s5k6aa: Add missing static storage class specifier

Fixes the following sparse warning:
drivers/media/video/s5k6aa.c:1439:5: warning:
symbol 's5k6aa_check_fw_revision' was not declared. Should it be static?
Signed-off-by: NSachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: NMauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/video/s5k6aa.c

@@ -1436,7 +1436,7 @@ static int s5k6aa_open(struct v4l2_subdev *sd, struct v4l2_subdev_fh *fh)
 	return 0;
 }
 
-int s5k6aa_check_fw_revision(struct s5k6aa *s5k6aa)
+static int s5k6aa_check_fw_revision(struct s5k6aa *s5k6aa)
 {
 	struct i2c_client *client = v4l2_get_subdevdata(&s5k6aa->sd);
 	u16 api_ver = 0, fw_rev = 0;