e1000e: fix unmap bug

This is in reference to https://bugzilla.redhat.com/show_bug.cgi?id=484494


Also addresses issue show in kerneloops

The e1000e transmit code was calling pci_unmap_page on dma handles that it
might have called pci_map_single on.

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>