maintainers: declare aquantia atlantic driver maintenance

Aquantia is resposible now for all new features and bugfixes.
Reflect that in MAINTAINERS.

Signed-off-by: Igor Russkikh <igor.russkikh@aquantia.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>