net: mvpp2: move the mii configuration in the ndo_open path

This moves the mii configuration in the ndo_open path, to allow handling
different mii configurations later and to switch between these
configurations at runtime.

Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Tested-by: Marcin Wojtas <mw@semihalf.com>
Signed-off-by: David S. Miller <davem@davemloft.net>