mlxsw: spectrum: Trap IPv4 DHCP packets in router

Currently, IPv4 DHCP packets are trapped during L2 forwarding, which
means that packets might be trapped unnecessarily. Instead, only trap
the DHCP packets that reach the router. Either because they were flooded
to the router port or forwarded to it by the FDB. This is consistent
with the corresponding IPv6 trap.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>