e1000: fix unmap bug

This is in reference to the issue shown in kerneloops (search e1000 unmap)

The e1000 transmit code was calling pci_unmap_page on dma handles that it
might have called pci_map_single on.

Same bug as e1000e

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Acked-by: Bruce Allan <bruce.w.allan@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>