net: phy: fix duplex out of sync problem while changing settings

As reported by Zhang there's a small issue if in forced mode the duplex
mode changes with the link staying up [0]. In this case the MAC isn't
notified about the change.

The proposed patch relies on the phylib state machine and ignores the
fact that there are drivers that uses phylib but not the phylib state
machine. So let's don't change the behavior for such drivers and fix
it w/o re-adding state PHY_FORCING for the case that phylib state
machine is used.

[0] https://lore.kernel.org/netdev/[e-mail]/T/

Fixes: 2bd229df5e2e ("net: phy: remove state PHY_FORCING")
Reported-by: Zhang Changzhong <[e-mail]>
Tested-by: Zhang Changzhong <[e-mail]>
Signed-off-by: Heiner Kallweit <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

drivers/net/phy/phy.c

···
 	phydev->mdix_ctrl = cmd->base.eth_tp_mdix_ctrl;
 
 	/* Restart the PHY */
-	_phy_start_aneg(phydev);
+	if (phy_is_started(phydev)) {
+		phydev->state = PHY_UP;
+		phy_trigger_machine(phydev);
+	} else {
+		_phy_start_aneg(phydev);
+	}
 
 	mutex_unlock(&phydev->lock);
 	return 0;