AX88179_178A: Add ethtool ops for EEE support

Add functions to support ethtool EEE manipulating, and the EEE
is disabled in default setting to enhance the compatibility
with certain switch.

Signed-off-by: Freddy Xin <freddy@asix.com.tw>
Signed-off-by: David S. Miller <davem@davemloft.net>