net: dsa: Relocate master ethtool operations

Relocate master_ethtool_ops and master_orig_ethtool_ops into struct
dsa_port in order to be both consistent, and make things self contained
within the dsa_port structure.

This is a preliminary change to supporting multiple CPU port interfaces.

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