net: dsa: mv88e6xxx: Add stats_get_stats to ops structure

Different families have different sets of statistics. Abstract this
using a stats_get_stats op. The mv88e6390 needs a different
implementation, which will be added later.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>