mlxsw: core: Add API to set trap action

Up until now the action of a trap was never changed during its lifetime.
This is going to change by subsequent patches that will allow devlink to
control the action of certain traps.

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