net: phy: allow driver to implement their own aneg_done

Some PHYs out there can be very quirky with respect to how they would
report the auto-negotiation is completed. Allow drivers to override the
generic aneg_done() implementation by providing their own.

Since not all drivers have been updated yet to use genphy_aneg_done() as
aneg_done() callback, we explicitely check that this callback is valid
before calling into it.

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