mlxsw: core: Expose generic macros for rx trap

In Spectrum, there is a macro to arrange the traps list.
This macro is useful for everyone who is using rx traps.
Create a similar macro in core.h for creating the generic listener struct
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>