i40e: Change unmatched function types

Change of function declaration from int to u64 due to
return type mismatch (u64).

Signed-off-by: Grzegorz Siwik <grzegorz.siwik@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>