mlxsw: spectrum_trap: Set unreg_action to be SET_FW_DEFAULT

Currently it does not really matter if it is set to DISCARD or
SET_FW_DEFAULT because it is set only during unregister of the listener.
The unreg_action is going to be used for disabling the listener too, so
change to SET_FW_DEFAULT and ensure the HW is going to behave correctly.

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