Major changes:
* Finally tracked down the flow control setting that
seems to have been causing TX stalls and watchdog timeouts
* RX and TX paths now share a lot more code
* TX interrupt is no longer used; we instead GC finished
tx queue entries at the bottom of the start routine.
* TX start now queues fragmented packets directly; it only
invokes defrag() for occasional very fragmented packets.
* "sysctl dev.cpsw" dumps controller statistics and queue counts
* Host Error Interrupt will give extensive debugging information
if the controller chokes on the queued data.
c2bbbcc61a