e100: Fix broken cbs accounting due to missing memset.

Alan Stern noticed that e100 caused slab corruption.
commit 98468efd changed
the allocation of cbs to use dma pools that don't return zeroed memory,
especially the cb->status field used to track which cb to clean, causing
(the visible) double freeing of skbs and a wrong free cbs count.

Now the cbs are explicitly zeroed at allocation time.

Reported-by: Alan Stern <stern@rowland.harvard.edu>
Tested-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Roger Oksanen <roger.oksanen@cs.helsinki.fi>
Acked-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>