igc: Fix TX timestamp support for non-MSI-X platforms

Time synchronization was not properly enabled on non-MSI-X platforms.

Fixes: 2c344ae2 ("igc: Add support for TX timestamping")
Signed-off-by: James McLaughlin <james.mclaughlin@qsc.com>
Reviewed-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Tested-by: Nechama Kraus <nechamax.kraus@linux.intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>