mlxsw: spectrum: Register for IPIP_DECAP_ERROR trap

These traps are generated for packets that fail checks for source IP,
encapsulation type, or GRE key. Trap these packets to CPU for follow-up
handling by the kernel, which will send ICMP destination unreachable
responses.

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