status registers for error conditions and updating statistics
when there are cycles left (inspired by the nge(4) driver).
- Removed the TX list counter and the producer/consumer gap; it's
enough to just ensure we don't reuse the last (free) descriptor,
as the chip may not have read its next pointer yet. If we reuse
it, the TX may stall under a heavy TX load with polling enabled.
- Dropped code to recharge the watchdog timer, it's pointless; the
watchdog routine will re-init the chip and both RX and TX lists.
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