i40e: clean up Tx fast path

Sync the fast path for i40e_tx_map and i40e_clean_tx_irq so that they
are similar to igb and ixgbe.

- Only update the Tx descriptor ring in tx_map
- Make skb mapping always on the first buffer in the chain
- Drop the use of MAPPED_AS_PAGE Tx flag
- Only store flags on the first buffer_info structure

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>