mlxsw: spectrum: Report link partner's advertised speeds

If autonegotiation was performed successfully, then we should report the
link partner's advertised speeds instead of the operational speed of the
port.

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>