Each TX queue can hold one packet (yes, if_emac can send only two(!)
packets at a time).
Even with this change the very limited FIFO buffer (3 KiB for TX and 13 KiB
for RX) fill up too quick to sustain higher throughput.
For the TCP case it turns out that TX isn't the limiting factor, but the RX
side is (the FIFO fill up and starts to discard packets, so the sender has
to slow down).
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