MAINTAINERS: Update mlx4_en driver entry

Remove Ido Shamay as co-maintainer for the mlx4 Ethernet driver,
as he no longer works for Mellanox.

Signed-off-by: Ido Shamay <idos@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>