net: phy: Ensure state transitions are processed from phy_stop()

In the phy_disconnect() -> phy_stop() path, we will be forcibly setting
the PHY state machine to PHY_HALTED. This invalidates the old_state !=
phydev->state condition in phy_state_machine() such that we will neither
display the state change for debugging, nor will we invoke the
link_change_notify() callback.

Factor the code by introducing phy_process_state_change(), and ensure
that we process the state change from phy_stop() as well.

Fixes: 5c5f626b ("net: phy: improve handling link_change_notify callback")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>