net: dsa: Pass dsa_port reference to ethtool setup/restore

We do not need to have a reference to a dsa_switch, instead we should
pass a reference to a CPU dsa_port, change that. This is a preliminary
change to better support multiple CPU ports.

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