igc: Remove stale comment about Tx timestamping

igc: Remove stale comment about Tx timestamping

The initial igc Tx timestamping implementation used only one register for
retrieving Tx timestamps. Commit 3ed247e78911 ("igc: Add support for
multiple in-flight TX timestamps") added support for utilizing all four of
them e.g., for multiple domain support. Remove the stale comment/FIXME.

Fixes: 3ed247e78911 ("igc: Add support for multiple in-flight TX timestamps")
Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>
Acked-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Tested-by: Naama Meir <naamax.meir@linux.intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>