ar71xx: fix failsafe acces on several devices

The ar71xx platform always uses the eth0 interface to provide
access to the device in failsafe mode. Due to this, failsafe
access is broken on devices where the LAN ports are connected
to the 'eth1' interface.

Update the relevant script to correctly set the ifname variable
for these devices.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

SVN-Revision: 39752