net: dsa: fully divert PHY reads/writes if requested

In case a PHY is found via Device Tree, and is also flagged by the
switch driver as needing indirect reads/writes using the switch driver
implemented MDIO bus, make sure that we bind this PHY to the slave MII
bus in order for this to happen.

Without this, we would succeed in having the PHY driver probe()'s
function to use slave MII bus read/write functions, because this is done
during dsa_slave_mii_init(), but past that point, the PHY driver would
not go through these diverted reads and writes.

Fixes: 0d8bcdd383b88 ("net: dsa: allow for more complex PHY setups")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>