net: dsa: split dsa_port's netdev member

The dsa_port structure has a "netdev" member, which can be used for
either the master device, or the slave device, depending on its type.

It is true that today, CPU port are not exposed to userspace, thus the
port's netdev member can be used to point to its master interface.

But it is still slightly confusing, so split it into more explicit
"master" and "slave" members inside an anonymous union.

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