net: dsa: mv88e6xxx: Factor out common initialization code

Code used and needed in mv886xxx.c should be initialized there as well,
so factor it out from the individual initialization files.

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