net: dsa: allow drivers to do link adjustment

Whenever libphy determines that the link status of a given PHY/port has
changed, allow to call into the switch driver link adjustment callback
so proper actions can be taken care of by the switch driver upon link
notification.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>