r6040: check MDIO register busy waiting result

We are currently busy waiting for MDIO registers to complete their
operation but we did not propagate the result back to the caller.
Update r6040_phy_{read,write} to report the busy waiting result
accordingly.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>