mdio add missing GPL flag

Add missing GPL flag and description.

mdio: module license 'unspecified' taints kernel.
Disabling lock debugging due to kernel taint

Signed-off-by: Nicolas Reinecke <nr <at> das-labor.org>
Acked-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>