mdio_bus: fix devm_mdiobus_alloc_size export

commit 6d48f44b "mdio_bus: implement devm_mdiobus_alloc/devm_mdiobus_free"
introduced a new function devm_mdiobus_alloc_size() but added an export
for a different function devm_mdiobus_alloc(), which was obviously
a simple mistake that leads to  build error whenever this function is
used from a loadable module:

ERROR: "devm_mdiobus_alloc_size" [drivers/net/ethernet/ti/davinci_mdio.ko] undefined!

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Grygorii Strashko <grygorii.strashko@ti.com>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Cc: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: netdev@vger.kernel.org
Acked-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>