i40iw: Fix potential fcn_id_array out of bounds

Avoid out of bounds error by utilizing I40IW_MAX_STATS_COUNT
instead of I40IW_INVALID_FCN_ID.

Signed-off-by: Christopher N Bednarz <christoper.n.bednarz@intel.com>
Signed-off-by: Henry Orosco <henry.orosco@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>