mlxsw: spectrum_router: Consolidate identical functions

After the last patch mlxsw_sp_fib{4,6}_node_entry_link() and
mlxsw_sp_fib{4,6}_node_entry_unlink() are identical and can therefore be
consolidated into the same common function.

Perform the consolidation.

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