net: dsa: mv88e6xxx: Implement mv88e6390 get_stats

The mv88e6390 uses a different bit to select between bank0 and bank1
of the statistics. So implement an ops function for this, and pass the
selector bit to the generic stats read function. Also, the histogram
selection has moved for the mv88e6390, so abstract its selection as
well.

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