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>