igc: Simplify setting flags in the TX data descriptor

We can re-use the IGC_SET_FLAG() macro to simplify setting some values
in the TX data descriptor. With the macro it's easier to get the
meaning of the operations.

Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Tested-by: Naama Meir <naamax.meir@linux.intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>