The reception code hands RX DMA descriptors back to the DMA right after its
contents were copied into the network stack internal buffer. This increases
the odds that the DMA never runs out of DMA descriptors to fill, even under
high load. However, the loop fetching the Ethernet frame stops to iterate at the
end of the frame. If the DMA used one more descriptor to store the FCS, this
was not returned back to the DMA. This commit fixes it.
45dc86acce