i40e: Disable link polling

Periodic link polling was added when the link events were found not to be
trustworthy.  This was the case early on, but was likely because the link
event mask was being used incorrectly.  As this has been fixed in recent
code, we can disable the link polling to lessen the AQ traffic.

Change-ID: Id890b5ee3c2d04381fc76ffa434777644f5d8eb0
Signed-off-by: Shannon Nelson <shannon.nelson@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>