sunxvr500: Ignore secondary output PCI devices.

These just represent the secondary and further heads attached to the
card, and they have different sets of PCI bar registers to map.

So don't try to drive them in the main driver.

Reported-by: Frans van Berckel <fberckel@xs4all.nl>
Tested-by: Frans van Berckel <fberckel@xs4all.nl>
Signed-off-by: David S. Miller <davem@davemloft.net>