mlxsw: spectrum: Keep maximum speed around

The maximum port speed depends on link modes supported by the port, and for
Ethernet ports is constant. The maximum speed will be handy when setting
SBIB, the internal buffer used for traffic mirroring. Therefore, keep it in
struct mlxsw_sp_port for easy access.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>