cxgb3: fix 2 ports 1G regression

commit 88045b3c
	cxgb3: fix mac index mapping

	Override the mac index computation for the gen2 adapter,
	as each port is expected to use index 0.

introduces a regression on 2 port 1G adapter
as its xauicfg vpd value is null.
Add a check on the device id.

Signed-off-by: Divy Le Ray <divy@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>