[media] v4l2: add const to argument of write-only s_frequency ioctl

This ioctl is defined as IOW, so pass the argument as const.

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

drivers/media/pci/cx18/cx18-ioctl.c

@@ -614,7 +614,7 @@ static int cx18_g_frequency(struct file *file, void *fh,
 	return 0;
 }
 
-int cx18_s_frequency(struct file *file, void *fh, struct v4l2_frequency *vf)
+int cx18_s_frequency(struct file *file, void *fh, const struct v4l2_frequency *vf)
 {
 	struct cx18_open_id *id = fh2id(fh);
 	struct cx18 *cx = id->cx;