fff-support: Update PoE passthrough code

This implements an upgrade safe solution for the PoE passthrough
on multiple devices. Since LEDE already provides the relevant
settings, a check whether those are in place enables both
identification of affected devices and automatic deactivation
if parameters are changed.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Tested-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Reviewed-by: Tim Niemeyer <tim@tn-x.org>