phy: micrel.c: Enable ksz9031 energy-detect power-down mode

Set bit 0 in register 1C.23 to enable the EDPD feature of the
KSZ9031 PHY. This reduces power consumption when the link is
down.

Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>