phylib: Fix auto-negotiation restart avoidance

A previous patch, 51e2a384, made
genphy_config_aneg() not restart aneg by calling genphy_restart_aneg() if
the advertisement hadn't changed.

But, genphy_restart_aneg() doesn't just restart aneg, it may also *enable*
aneg or un-isolate the PHY from the MII (those functions are controlled by
the same register).  The code to avoid calling genphy_restart_aneg() didn't
consider this.

So, modify genphy_config_aneg() to also check if the PHY needs to have aneg
enabled or be un-isolated before deciding not to restart aneg.

This caused a problem with certain Davicom PHYs, as that driver isolates
the PHY (why?) before calling genphy_config_aneg() and expects the PHY to
be un-isolated by that function.

Signed-off-by: Trent Piepho <tpiepho@freescale.com>
Reported-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>