net/phy: add missing kernel-doc

Fix kernel-doc warning, missing description:

Warning(lin2627-g3-kdocfixes//drivers/net/phy/mdio_bus.c:63): No description found for parameter 'd'
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/phy/mdio_bus.c

@@ -55,6 +55,7 @@ EXPORT_SYMBOL(mdiobus_alloc);
 
 /**
  * mdiobus_release - mii_bus device release callback
+ * @d: the target struct device that contains the mii_bus
  *
  * Description: called when the last reference to an mii_bus is
  * dropped, to free the underlying memory.