scripts: fix lan/wan order of malta in qemustart

scripts: fix lan/wan order of malta in qemustart

OpenWrt's 99-default_network assigns eth0 to lan and eth1 to wan
when no target-specific 02_network is present, which is the case
for malta.  The qemustart bridge block however placed the wan
-device before the lan -device, so the guest's eth0 (lan) ended
up attached to $BR_WAN and eth1 (wan) to $BR_LAN.

Swap the order to match the guest's actual role assignment.

Link: https://github.com/openwrt/openwrt/pull/23424
Signed-off-by: Paul Spooren <mail@aparcar.org>