mlxsw: core: Introduce generic macro for event

Create a macro for creating the generic listener struct for events,
similar to the one for rx traps.

Signed-off-by: Nogah Frankel <nogahf@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>