mlxsw: spectrum_router: Refactor nexthop init routine

The nexthop init and de-init functions both have symmetric parts
concerned with the reflection of the neighbour entry into the device's
adjacency table, in case it's used by a gatewayed route.

These sections of code also need to be called when a nexthop is marked
as valid / invalid following NH_{ADD,DEL} events. Break these out into
appropriate functions, so that they could be invoked following the
reception of above events.

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