net: libphy: Add stubs to hook IEEE MMD Register reads and writes

net: libphy: Add stubs to hook IEEE MMD Register reads and writes

The Micrel ksz9021 PHY does not support standard IEEE standard MMD
extended register access, therefore requires stubs to fail the read
register method and do nothing for the write register method when
libphy attempts to read and/or configure Energy Efficient Ethernet
features in PHYS that do support those features. This problem
was observed on an Altera Cyclone V SOC development kit that
uses the Synopsys EMAC and the Micrel ksz9021 PHY. This patch
was tested on the same board, and Energy Efficient Ethernet is
now disabled as expected since the Micrel PHY does not support that
feature.

Signed-off-by: Vince Bridgers <vbridgers2013@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>