net: bcmgenet: connect and disconnect from the PHY state machine

phy_disconnect() is the only way to guarantee that we are not going to
schedule more work on the PHY state machine workqueue for that
particular PHY device.

This fixes an issue where a network interface was suspended prior to a
system suspend/resume cycle and would then be resumed as part of
mdio_bus_resume(), since the GENET interface clocks would have been
disabled, this basically resulted in bus errors to appear since we are
invoking the GENET driver adjust_link() callback.

Fixes: b6e978e5 ("net: bcmgenet: add suspend/resume callbacks")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>