mlxsw: spectrum: Keep maximum MTU around

The maximum port MTU depends on port type. On Spectrum, mlxsw configures
all ports as Ethernet ports, and the maximum MTU therefore never changes.
Besides checking MTU configuration, maximum MTU will also 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>