net: dsa: mv88e6xxx: Allow the SERDES interfaces to have statistics

When gettting the number of statistics, the strings and the actual
statistics, call the SERDES ops if implemented. This means the stats
code needs to return the number of strings/stats they have placed into
the data, so that the SERDES strings/stats can follow on.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>