IB/mlx4: Enable 4K mtu for IBoE

The IBoE port MTU is derived from the corresponding Ethernet netdevice
MTU, which can support jumbo frames of 9K, and hence surely supports
the max IB mtu of 4K.

Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>