net/mlx5e: Max mtu comparison fix

On change mtu the driver compares between hardware queried mtu and
software requested mtu. We need to compare between software
representation of the queried mtu and the requested mtu.

Fixes: facc9699 ('net/mlx5e: Fix HW MTU settings')
Signed-off-by: Doron Tsur <doront@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>