atl1: fix 4G memory corruption bug

When using 4+ GB RAM and SWIOTLB is active, the driver corrupts
memory by writing an skb after the relevant DMA page has been
unmapped.  Although this doesn't happen when *not* using bounce
buffers, clearing the pointer to the DMA page after unmapping
it fixes the problem.

http://marc.info/?t=120861317000005&r=2&w=2

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Jay Cliburn <jacliburn@bellsouth.net>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>