i40e/i40evf: fix a potential type compare issue

Rework an if expression to assure there is no type compare problem between
a size and a possible negative number.

Change-ID: I4921fcc96abfcf69490efce020a9e4007f251c99
Reported-by: Helin Zhang <helin.zhang@intel.com>
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>