Documentation: mdio_bus.c - fix warnings

Fix wrong parameter description and related warnings at 'make htmldocs'.

Signed-off-by: Lothar Rubusch <l.rubusch@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: David S. Miller <davem@davemloft.net>