Add DSA helpers to setup and teardown a master net device wired to its
CPU port. This centralizes the dsa_ptr assignment.

This also makes the master ethtool helpers static at the same time.
Signed-off-by: NVivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>