igb: cleanup igb xmit frame path

This patch cleans up the xmit frame path for igb to better handle xmit
frame errors and avoid null pointer exceptions.  It also cleans up some
whitespace issues found in the xmit frame path.

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>