mpc85xx: fix failsafe iface selection for mpc85xx boards

mpc85xx: fix failsafe iface selection for mpc85xx boards

Some mpc85xx boards still boot with failsafe configured on a non-LAN
interface. Align the preinit interface with the first DSA port or the
interface that LAN is connected to.

This makes failsafe reachable on devices where the default selection does
not map to the primary LAN port.

Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/21516
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
(cherry picked from commit a73db6d0a041e1b86bd72ee4103b23d41887a31e)