ixgb: refactor tx path to use skb_dma_map/unmap

This code updates ixgb so that it can use the skb_dma_map/unmap functions
to map the buffers.  In addition it also updates the tx hang logic to use
time_stamp instead of dma to determine if it has detected a tx hang.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>