kernel-doc: fix new warning in net/phy/mdio_bus.c

Fix new kernel-doc warning:

Warning(drivers/net/phy/mdio_bus.c:49): No description found for parameter 'size'

Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: David S. Miller <davem@davemloft.net>