drivers: net: loopback: Do not change the original pkt

When we are sending a network pkt, do not tweak the original
packet but the cloned one. The original behavior is ok too, but
logically we should adjust the cloned packet only that is being
received by the stack. This also means that we avoid one extra
copy to tmp variable when sending the packet.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>