net/mlx4: Replace mlx4_en_mac_to_u64() with mlx4_mac_to_u64()

Currently, the EN driver uses a private static function
mlx4_en_mac_to_u64(). Move it to a common include file (driver.h)
for mlx4_en and mlx4_ib for further use.

Signed-off-by: Eugenia Emantayev <eugenia@mellanox.com>
Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>