Under "heavy" RX load, the driver cannot handle the descriptors fast
enough. In detail, when a descriptor is consumed, its used flag is
cleared and once the RX budget is consumed all descriptors with a
cleared used flag are prepared to receive more data. Under load though,
the HW may constantly receive more data and use those descriptors with a
cleared used flag before they are actually prepared for next usage.

The head and tail pointers into the RX-ring should always be valid and
we can omit clearing and checking of the used flag.
Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: David S. Miller <davem@davemloft.net>