MAINTAINERS: add davem in NETWORKING DRIVERS

"./scripts/get_maintainer.pl -f" does not actually show us David as the
maintainer of drivers/net directories such as team, bonding, phy or dsa.
Adding him in an M: entry of NETWORKING DRIVERS fixes this.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>