net/mlx4_en: Set correct port parameters during device initialization

Set valid port parameters: MTU and flow control configuration when
configuring the port during HW device initialization,
prior to the net device open() being called.
Using  invalid parameters (such as all zeros)
could lead to bad firmware behavior.

Signed-off-by: Yevgeny Petrilin <yevgenyp@mellanox.co.il>
Signed-off-by: David S. Miller <davem@davemloft.net>