net: r6040: Return proper error for r6040_init_one

Return -ENOMEM instead of 0 for the case of mdiobus_alloc and kmalloc failure.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>