igc: Fix not considering the TX delay for timestamps

When timestamping a packet there's a delay between the start of the
packet and the point where the hardware actually captures the
timestamp. This difference needs to be considered if we want accurate
timestamps.

This was done on the RX side, but not on the TX side.

Fixes: 2c344ae2

 ("igc: Add support for TX timestamping")
Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>