i40e: Reorder logic for coalescing RS bits

This patch reorders the logic at the end of i40e_tx_map to address the
fact that the logic was rather convoluted and much larger than it needed
to be.

In order to try and coalesce the code paths I have updated some of the
comments and repurposed some of the variables in order to reduce
unnecessary overhead.

This patch does the following:
1.  Quit tracking skb->xmit_more with a flag, just max out packet_stride
2.  Drop tail_bump and do_rs and instead just use desc_count and td_cmd
3.  Pull comments from ixgbe that make need for wmb() more explicit.

Change-ID: Ic7da85ec75043c634e87fef958109789bcc6317c
Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>