net: bcmgenet: Power on integrated GPHY in bcmgenet_power_up()

We are currently disabling the GPHY interface during bcmgenet_close(),
and attempting to power it back on during bcmgenet_open(). This works
fine for the first time, because we called bcmgenet_mii_config() which
took care of enabling the interface, however, bcmgenet_power_up() really
needs to power on the GPHY for correctness.

This will be particularly important as we want to move
bcmgenet_mii_probe() down to bcmgenet_open() to avoid seeing the "PHY
already attached" message.

Fixes: a642c4f7 ("net: bcmgenet: power up and down integrated GPHY when unused")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>