ar71xx: Error fix in Kernel for MR3420v2 Ethernet Switch configuration

ar71xx: Error fix in Kernel for MR3420v2 Ethernet Switch configuration

Error in using another MDIO (AP123 platform using only one GE1 device
for eth0 and eth1) for the router mr3420v2.
?onfirmed: https://forum.openwrt.org/viewtopic.php?pid=194779#p194779

Routers in this sense are the same and the only difference is: USB
port, USB led, instead of the GPIO-switch we have GPIO-button.

Signed-off-by: Dmytro <dioptimizer@gmail.com>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
SVN-Revision: 36036