mlxsw: spectrum: Use action 'discard' when removing traps

When removing packet traps we should use action 'discard' instead of
'forward', as some trap IDs we'll add cannot be configured with the
later. However, result is the same, as packets are not trapped to the
CPU.

In the future we will be able to reverse the operation properly by
detaching the trap group from the CPU.

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