net: phy: smsc: Re-enable EDPD mode for LAN87xx

This patch re-enables Energy Detect Power Down (EDPD) mode for the
LAN8710/LAN8720.  EDPD mode was disabled in a previous commit,
(b629820d), because it was causing the
PHY to not be able to detect a link when cold started without a cable
connected.

The LAN8710/LAN8720 requires a minimum of 2 link pulses within 64ms of
each other in order to set the ENERGYON bit and exit EDPD mode.  If a
link partner does send the pulses within this interval, the PHY will
remained powered down.

This workaround will manually toggle the PHY on/off upon calls to
read_status in order to generate link test pulses if the link is down.
If a link partner is present, it will respond to the pulses, which will
cause the ENERGYON bit to be set and will cause the EDPD mode to be
exited.

Signed-off-by: Patrick Trantham <patrick.trantham@fuel7.com>
Signed-off-by: David S. Miller <davem@davemloft.net>