net: axienet: Handle 0 packet receive gracefully

The AXI-DMA rx-delay interrupt can sometimes be triggered
when there are 0 outstanding packets received. This is due
to the fact that the receive function will greedily consume
as many packets as possible on interrupt. So if two packets
(with a very particular timing) arrive in succession they
will each cause the rx-delay interrupt, but the first interrupt
will consume both packets.
This means the second interrupt is a 0 packet receive.

This is mostly OK, except that the tail pointer register is
updated unconditionally on receive. Currently the tail pointer
is always set to the current bd-ring descriptor under
the assumption that the hardware has moved onto the next
descriptor. What this means for length 0 recv is the current
descriptor that the hardware is potentially yet to use will
be marked as the tail. This causes the hardware to think
its run out of descriptors deadlocking the whole rx path.

Fixed by updating the tail pointer to the most recent
successfully consumed descriptor.

Reported-by: Wendy Liang <wendy.liang@xilinx.com>
Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Tested-by: Jason Wu <huanyu@xilinx.com>
Acked-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: David S. Miller <davem@davemloft.net>