mini_snmpd: fix initscript to use l3_device

mini_snmpd: fix initscript to use l3_device

for working out the interface name
Working:
ubus -S call network.interface dump|jsonfilter -e "@.interface[@.interface=\"wan\"].l3_device"
Broken:
ubus -S call network.interface dump|jsonfilter -e "@.interface[@.interface=\"wan\"].device"

Fix run tested:
root@wifi:/overlay/upper# ps |grep mini_snmpd
 1404 root       980 S    /usr/bin/mini_snmpd -n -c public -L Undisclosed -C VGB <admin@victimsofgaybullying.com> -t 1 -a -d /overlay,/tmp -i br-lan,pppoe-w

Before it wasn't using the pppoe interface it was using the parent
interface eth0 twice. Small 1 line fix. Merge at your convenience.

Signed-off-by: Luke McKee <hojuruku@gmail.com>