rocker: Handle protodown notifications.

protodown can be set by user space applications like MLAG on detecting
errors on a switch port. This patch provides sample switch driver changes
for handling protodown. Rocker PHYS disables the port in response to
protodown.

Signed-off-by: Anuradha Karuppiah <anuradhak@cumulusnetworks.com>
Signed-off-by: Andy Gospodarek <gospo@cumulusnetworks.com>
Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: Wilson Kok <wkok@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>