net: dsa: mv88e6xxx: Add mv88e6390 statistics unit init

The statistics unit on the mv88e6390 needs the histogram mode to be
configured in a different register compared to other devices. Add an
ops to do this.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
v2:
  Rename to mv88e6390_g1_stats_set_histogram
  Move into global1.c
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/dsa/mv88e6xxx/global1.h

@@ -22,5 +22,6 @@ int mv88e6xxx_g1_wait(struct mv88e6xxx_chip *chip, int reg, u16 mask);
 int mv88e6xxx_g1_stats_snapshot(struct mv88e6xxx_chip *chip, int port);
 int mv88e6320_g1_stats_snapshot(struct mv88e6xxx_chip *chip, int port);
 int mv88e6390_g1_stats_snapshot(struct mv88e6xxx_chip *chip, int port);
+int mv88e6390_g1_stats_set_histogram(struct mv88e6xxx_chip *chip);
 
 #endif /* _MV88E6XXX_GLOBAL1_H */