dsa/mv88e6060: support nonzero mii base address

The mv88e6060 uses either the lower 16 or upper 16 mii addresses,
depending on the value of the EE_CLK/ADDR4 pin. Support both
configurations by using the sw_addr setting as base address.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>
Acked-by: Lennert Buytenhek <buytenh@wantstofly.org>
Signed-off-by: David S. Miller <davem@davemloft.net>