video: ffb: fix ffb_probe error path

Current implementation calls of_iounmap for par->fbc twice in error path.
In the case of goto out_unmap_dac, we should call of_iounmap for par->dac.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>