iavf: Refactor string format to avoid static analysis warnings

Change format to match variable type that is used in string.

Use %u format for unsigned variable and %d format for signed variable
to remove static analysis warnings.

Signed-off-by: Michal Swiatkowski <michal.swiatkowski@intel.com>
Signed-off-by: Karen Sornek <karen.sornek@intel.com>
Tested-by: George Kuruvinakunnel <george.kuruvinakunnel@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>