mlxsw: Make system port to local port mapping explicit

System ports are unique identifiers in a multi-ASIC environment that
represent all the available ports in the system. Local ports on the
other hand, are unique only within the local ASIC.

Since system port to local port mapping is not part of the HW-SW
contract and since only single-ASIC configurations are currently
supported, set an explicit 1:1 mapping by configuring the Switch System
Port Record (SSPR) register.

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>