i40e: Cleanup Tx buffer info layout

- drop the mapped_as_page u8 from the Tx buffer info as it was unused
- use the DMA unmap accessors for Tx DMA
- replace checks of DMA with checks of the unmap length to verify if an
  unmap is needed
- update the Tx buffer layout to make it consistent with igb, ixgbe

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Kavindya Deegala <kavindya.s.deegala@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>