netxen: fix incorrect loop counter decrement

The loop counter k is currently being decremented from zero which
is incorrect. Fix this by incrementing k instead

Detected by CoverityScan, CID#401847 ("Infinite loop")

Fixes: 83f18a55 ("netxen_nic: fw dump support")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>