net: dsa: mv88e6xxx: Fix stats histogram mode

The statistics histogram mode was not being explicitly initialized on
devices other than the 6390 family. Clearing the statistics then
overwrote the default setting, setting the histogram to a reserved
mode.

Explicitly set the histogram mode for all devices. Change the
statistics clear into a read/modify/write, and since it is now more
complex, move it into global1.c.

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