Dump out the holding buffer descriptor contents and addresses stopping DMA.
Limit the number of software queued frames when doing non-aggregation.

This should prevent the TX queue being filled with non-aggregate frames,
causing starvation and non-fair queue behaviour.
Tidy up the debugging - don't bother printing out TID pointers; now
that we are printing out the MAC address in these fields, just printing
out the TID is enough.
A library function shall not set errno to 0.

Reviewed by:	mdf