via-rhine: VTunknown1 device is really VT8251 South Bridge

The VIA Technologies VT8251 South Bridge's integrated Rhine-II
Ethernet MAC comes has a PCI revision value of 0x7c.  This was
verified on ASUS P5V800-VM mainboard.

Signed-off-by: Kevin Brace <kevinbrace@bracecomputerlab.com>
Signed-off-by: David S. Miller <davem@davemloft.net>