dp8393x: Update LLFA and CRDA registers from rx descriptor

dp8393x: Update LLFA and CRDA registers from rx descriptor

Follow the algorithm given in the National Semiconductor DP83932C
datasheet in section 3.4.7:

    At the next reception, the SONIC re-reads the last RXpkt.link field,
    and updates its CRDA register to point to the next descriptor.

The chip is designed to allow the host to provide a new list of
descriptors in this way.

Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Tested-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>