igb: Update igb Tx flags to improve code efficiency

This change is meant to improve the efficiency of the Tx flags in igb by
aligning them with the values that will later be written into either the
cmd_type or olinfo.  By doing this we are able to reduce most of these
functions to either just a simple shift followed by an or in the case of
cmd_type, or an and followed by an or in the case of olinfo.

In order to avoid type conversion errors I also adjusted the locations
where we were switching between CPU and little endian.

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