Hooking the transmit queue enqueue is not what I would have expected at all. 
That makes it appear that the packet went on the wire when the packet was
enqueued.  The packet is actually sent after it is dequeued.

I think the helper should hook the dequeue trace hook for pcap tracing since
there can be a significant delta-t between the enqueue and the dequeue.

Perhaps even better, there should be a new low-level trace hook Tx that
corresponds to a packet being sent out to the channel.  Right now there is a
hole where a packet can be dequeued from the transmit queue and subsequently
silently dropped.  This makes it appear as if the packet was transmitted
successfully.

I think it would be better to have upper level trace hooks into the transmit
queue, and lower level trace hooks at the device/channel interface.  Right now
things are mixed and somewhat confused/confusing.