net/mlx4_en: Check the enabling mtu flag in SET_PORT wrapper flow

Make sure MTU mask flag is set using new field upon set port
request. In addition, move this code into a helper function for better
code readability.

Signed-off-by: Shaker Daibes <shakerd@mellanox.com>
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>