Controller does not require TX start command for every frame. So

send a single TX command after setting up all TX frames.  This
removes unnecessary register accesses and bus_dmamap_sync(9) calls.
et(4) uses TX interrupt moderation so it's possible to have TX
buffers that were already transmitted but waiting for TX completion
interrupt.  If the number of available TX descriptor is less then
1/3 of total TX descriptor, try reclaiming first to get enough free
TX descriptors before setting up TX descriptors.
After r228325, et_txeof() no longer tries to send frames after
reclaiming TX buffers.  That change was made to give more chance
to transmit frames in main interrupt handler since we can still
send frames in interrupt handler with RX interrupt.  So right
before exiting interrupt hander, after enabling interrupt, try to
send more frames.  This gives slightly better performance numbers.

While I'm here reduce number of spare TX descriptors from 8 to 4.
Controller does not require reserved TX descriptors, it was just to
reduce TX overhead.  After r228325, driver has much lower TX
overhead so it does not make sense to reserve 8 TX descriptors.

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