mlxsw: spectrum_router: Introduce nexthop action field

Currently, the action associated with the nexthop is assumed to be
'forward' unless the 'discard' bit is set.

Instead, simplify this by introducing a dedicated field to represent the
action of the nexthop. This will allow us to more easily introduce more
actions, such as trap.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>