mlxsw: spectrum: Remember split base local port and use it in unsplit

Don't compute the original base local port during unsplit, rather
remember it in mlxsw_sp_port structure during split port creation.

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