i40iw: Remove check on return from device_init_pestat()

Remove unnecessary check for return code from
device_init_pestat() and change func to void.

Signed-off-by: Shiraz Saleem <shiraz.saleem@intel.com>
Signed-off-by: Henry Orosco <henry.orosco@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>