net: dsa: mv88e6xxx: Add mv88e6390 stats snapshot operation

The MV88E6390 has a control register for what the histogram statistics
actually contain. This means the stat_snapshot method should not set
this information. So implement the 6390 stats_snapshot function without
these bits.

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