net: dsa: Consolidate getting the statistics

Reading the statistics from the hardware is the same for all
chips. What differs is the number of available statistics. Have just
one copy of the code in the shared mv88e6xxx.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: David S. Miller <davem@davemloft.net>