net: dsa: setup and teardown master device

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