net: phy: add and use phy_check_downshift

So far PHY drivers have to check whether a downshift occurred to be
able to notify the user. To make life of drivers authors a little bit
easier move the downshift notification to phylib. phy_check_downshift()
compares the highest mutually advertised speed with the actual value
of phydev->speed (typically read by the PHY driver from a
vendor-specific register) to detect a downshift.

v2:
- Add downshift hint to phy_print_status

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/phy.h

@@ -365,6 +365,7 @@ struct macsec_ops;
  * suspended_by_mdio_bus: Set to true if this phy was suspended by MDIO bus.
  * sysfs_links: Internal boolean tracking sysfs symbolic links setup/removal.
  * loopback_enabled: Set true if this phy has been loopbacked successfully.
+ * downshifted_rate: Set true if link speed has been downshifted.
  * state: state of the PHY for management purposes
  * dev_flags: Device-specific flags used by the PHY driver.
  * irq: IRQ number of the PHY's interrupt (-1 if none)
@@ -405,6 +406,7 @@ struct phy_device {
 	unsigned suspended_by_mdio_bus:1;
 	unsigned sysfs_links:1;
 	unsigned loopback_enabled:1;
+	unsigned downshifted_rate:1;
 
 	unsigned autoneg:1;
 	/* The most recently read link state */
@@ -698,6 +700,7 @@ static inline bool phy_is_started(struct phy_device *phydev)
 
 void phy_resolve_aneg_pause(struct phy_device *phydev);
 void phy_resolve_aneg_linkmode(struct phy_device *phydev);
+void phy_check_downshift(struct phy_device *phydev);
 
 /**
  * phy_read - Convenience function for reading a given PHY register