net: dsa: lan9303: calculate offload_fwd_mark from tag

The lan9303 set bits in the host CPU tag indicating if a ingress frame
is a trapped IGMP or STP frame. Use these bits to calculate
skb->offload_fwd_mark more efficiently.

Signed-off-by: Egil Hjelmeland <privat@egil-hjelmeland.no>
Signed-off-by: David S. Miller <davem@davemloft.net>