i40e: only warn once of PTP nonsupport in 100Mbit speed

Only warn once that PTP is not supported when linked at 100Mbit.

Yes, using a static this way means that this once-only message is not
port specific, but once only for the life of the driver, regardless of
the number of ports.  That should be plenty.

Change-ID: Ie6476530056df408452e195ef06afd4f57caa4b2
Signed-off-by: Shannon Nelson <shannon.nelson@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>