net: dsa: move master ethtool code

DSA overrides the master device ethtool ops, so that it can inject stats
from its dedicated switch CPU port as well.

The related code is currently split in dsa.c and slave.c, but it only
scopes the master net device. Move it to a new master.c DSA core file.

This file will be later extented with master net device specific code.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>