MAINTAINERS: Update Mellanox's Eth NIC driver entries

Eugenia (Jenny) Emantayev is replacing Amir Vadai as the
mlx4 Ethernet driver maintainer.

Saeed Mahameed is assigned to maintain mlx5 Eth functionality.

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