lan78xx: PHY DSP registers initialization to address EEE link drop issues with long cables

The patch is to configure DSP registers of PHY device
to handle Gbe-EEE failures with >40m cable length.

Fixes: 55d7de9d ("Microchip's LAN7800 family USB 2/3 to 10/100/1000 Ethernet device driver")
Signed-off-by: Raghuram Chary J <raghuramchary.jallipalli@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>