igc: Rename IGC_TSYNCTXCTL_VALID macro

Rename the IGC_TSYNCTXCTL_VALID macro to IGC_TSYNCTXCTL_TXTT_0 so it
matches the datasheet.

Signed-off-by: Andre Guedes <andre.guedes@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>