Bluetooth: Mesh: Fixes seg_tx_reset adv buf unref

In seg_tx_reset() in transport.c, set the busy flag to 0U
before doing adv buf unref, which will avoid sending
unnecessary adv packets in case the adv buf is already put
in the mesh adv_queue.

Fixes #20970

Signed-off-by: Maximus Liu <maximus.liu@gmail.com>