ixgbe: Minor formatting and comment corrections for ixgbe_xmit_frame_ring

This patch is meant to address several minor issues in
ixgbe_xmit_frame_ring.  Specifically it adds a comment explaining the TXSW
flag, and correctly wraps a line over 80 characters.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>