net: phy: vitesse: implement downshift in vsc73xx phys

This commit implements downshift feature in vsc73xx family phys.

By default downshift was enabled with maximum tries.

Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>