net: dsa: Add initialization helper for CPU port ethtool_ops

Add a helper function: dsa_cpu_port_ethtool_init() which initializes a
custom ethtool_ops structure with custom DSA ethtool operations for CPU
ports. This is a preliminary change to move the initialization outside
of net/dsa/slave.c.

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>