net/sfc: implement port representor link update

net/sfc: implement port representor link update

Implement the callback by reporting link down if the representor
is not started, otherwise report link up with undefined link speed.

Link speed is undefined since representors can pass traffic to each
other even if the PF link is down.

Signed-off-by: Igor Romanov <igor.romanov@oktetlabs.ru>
Signed-off-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Reviewed-by: Andy Moreton <amoreton@xilinx.com>
Reviewed-by: Ivan Malov <ivan.malov@oktetlabs.ru>