drivers: net: xgene: XFI PCS reset when link is down

This patch fixes the link recovery issue, by doing PCS reset
when the link is down.

Signed-off-by: Fushen Chen <fchen@apm.com>
Signed-off-by: Iyappan Subramanian <isubramanian@apm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>