korina: periodically poll the media

This patch makes the korina driver poll the media
for link change. This is actually required on
Mikrotik RB532 (not RB532A) for korina to
operate properly.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>