net: dsa: Pass stringset to ethtool operations

Up until now we largely assumed that we were interested in ETH_SS_STATS
type of strings for all ethtool operations, this is about to change with
the introduction of additional string sets, e.g: ETH_SS_PHY_STATS.
Update all functions to take an appropriate stringset argument and act
on it when it is different than ETH_SS_STATS for now.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>