[media] fsl-viu: fix compiler warning

drivers/media/platform/fsl-viu.c: In function 'vidioc_s_fbuf':
drivers/media/platform/fsl-viu.c:867:32: warning: initialization discards 'const' qualifier from pointer target type [enabled by default]
This is fall-out from this commit:
commit e6eb28c
Author: Hans Verkuil <hans.verkuil@cisco.com>
    [media] v4l2: make vidioc_s_fbuf const

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Anatolij Gustschin <agust@denx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/platform/fsl-viu.c

@@ -864,7 +864,7 @@ int vidioc_s_fbuf(struct file *file, void *priv, const struct v4l2_framebuffer *
 {
 	struct viu_fh  *fh = priv;
 	struct viu_dev *dev = fh->dev;
-	struct v4l2_framebuffer *fb = arg;
+	const struct v4l2_framebuffer *fb = arg;
 	struct viu_fmt *fmt;
 
 	if (!capable(CAP_SYS_ADMIN) && !capable(CAP_SYS_RAWIO))