mlx4_en: Added missing iounmap upon releasing a device

Fixed a memory leak caused by missing iounmap when device
is being released.

Signed-off-by: Alexander Guller <alexg@mellanox.co.il>
Signed-off-by: Sharon Cohen <sharonc@mellanox.co.il>
Signed-off-by: David S. Miller <davem@davemloft.net>