When the list of free RX descriptors fills up completely, the consumed

When the list of free RX descriptors fills up completely, the consumed
and produced index are equal.  The RX refill operation did not refill
in that case, because it believed that this means there's nothing to
do.  In that case no RX buffers were allocated, and the RX descriptor
ring went dry forever.  Instead loop until we reach an empty freelist
entry.