net: dsa: Keep the mii bus and address in the private structure

Rather than looking up the mii bus and address every time, do it once
at probe, and keep it in the private structure. Centralise this probe
code in mv88e6xxx.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Tested-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>