dp8393x: Don't clobber packet checksum

dp8393x: Don't clobber packet checksum

A received packet consumes pkt_size bytes in the buffer and the frame
checksum that's appended to it consumes another 4 bytes. The Receive
Buffer Address register takes the former quantity into account but
not the latter. So the next packet written to the buffer overwrites
the frame checksum. Fix this.

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