net: phy: add genphy_aneg_done()

In preparation for allowing PHY drivers to potentially override their
auto-negotiation done callback, move the contents of phy_aneg_done() to
genphy_aneg_done() since that function really is the generic
implementation based on the BMSR_ANEGCOMPLETE status.

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