allocate a static buffer, which is added at the end of short packets
to pad the buffer to ETHER_MIN_LEN - ETHER_CRC_LEN.
While I'm there fix 2 bugs:
in qeinit(), unload the right dmamap if bus_dmamap_load fails
in qestart, don't dmamap_load the mbuf if its len is 0.
Tested on simh-vax.