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>