i40e: fix PTP on 5Gb links

As reported by Alex Sergeev, the i40e driver is incrementing the PTP
clock at 40Gb speeds when linked at 5Gb. Fix this bug by making
sure that the right multiplier is selected when linked at 5Gb.

Fixes: 3dbdd6c2 ("i40e: Add support for 5Gbps cards")
Cc: stable@vger.kernel.org
Reported-by: Alex Sergeev <asergeev@carbonrobotics.com>
Suggested-by: Alex Sergeev <asergeev@carbonrobotics.com>
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Tony Brelinski <tonyx.brelinski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>