mlxsw: Trap ARP packets at layer 3 instead of layer 2

Currently, the traps 'ARP_REQUEST' and 'ARP_RESPONSE' occur at layer 2.
To allow the packets to be flooded, they are configured with the action
'MIRROR_TO_CPU' which means that the CPU receives a replica of the packet.

Today, Spectrum ASICs also support trapping ARP packets at layer 3. This
behavior is better, then the packets can just be trapped and there is no
need to mirror them. An additional motivation is that using the traps at
layer 2, the ARP packets are dropped in the router as they do not have an
IP header, then they are counted as error packets, which might confuse
users.

Add the relevant traps for layer 3 and use them instead of the existing
traps. There is no visible change to user space.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>