[TG3]: Fix Sun tg3 variant detection.

Some Sun parts don't have PCI_VENDOR_ID_SUN in the subsystem
vendor ID.  So add another fallback test, which is the name
of the OBP firmware device tree node.  If it's a Sun part we'll
get "network", else it will be named "ethernet".

Signed-off-by: David S. Miller <davem@davemloft.net>