dl2k: nulify fraginfo after unmap

Patch fixes: "DMA-API: device driver tries to free an invalid DMA
memory address" warning reported here:
https://bugzilla.redhat.com/show_bug.cgi?id=639824



Reported-by: Frantisek Hanzlik <franta@hanzlici.cz>
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>