r6040: use an unique MDIO bus name

We should use an unique MDIO bus name which does not clash with anything
else in the system like the Fixed MDIO bus. The bus is now named:
r6040-<card number> which is unique in the system.

Reported-by: Vladimir Kolpakov <vova.kolpakov@gmail.com>
Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>