xgbe: switch to more generic VxLAN detection

Instead of looping though the list of ports just check
if the geometry of the packet is correct for VxLAN.
HW most likely doesn't care about the exact port, anyway,
since only first port is actually offloaded, and this way
we won't have to maintain the port list at all.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>