net: phy: provide a hook for link up/link down events

Sometimes, we need to do additional work between the PHY coming up and
marking the carrier present - for example, we may need to wait for the
PHY to MAC link to finish negotiation.  This changes phylib to provide
a notification function pointer which avoids the built-in
netif_carrier_on() and netif_carrier_off() functions.

Standard ->adjust_link functionality is provided by hooking a helper
into the new ->phy_link_change method.

Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>