MAINTAINERS: Update mlx4_en entry

Add Ido Shamay as co-maintainer for the mlx4 Ethernet driver.

Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>