net: dsa: bcm_sf2: Turn on PHY to allow successful registration

We are binding to the PHY using the SF2 slave MDIO bus that we create,
binding involves reading the PHY's MII_PHYSID1/2 which won't be possible
if the PHY is turned off. Temporarily turn it on/off for the bus probing
to succeeed. This fixes unbind/bind problems where the port connecting
to that PHY would be in error since it could not connect to it.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>