mlxsw: Use the same maximum MTU value throughout the driver

Currently, the driver uses two different values for maximum MTU, one is
stored in mlxsw_port->dev->max_mtu and the second is stored in
mlxsw_port->max_mtu. The second one is set to value which is queried from
firmware. This value was never tested, and unfortunately is not really
supported. That means that with the existing code, user can set MTU to
X, which is not really supported by firmware and which is bigger than
buffer size which is allocated in pci.

To make the driver consistent, use only mlxsw_port->dev->max_mtu for
maximum MTU value, for buffers headroom add Ethernet frame headers, which
are not included in mlxsw_port->dev->max_mtu. Remove mlxsw_port->max_mtu.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Link: https://lore.kernel.org/r/89fa6f804386b918d337e736e14ac291bb947483.1718275854.git.petrm@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>