i40iw: Remove NULL check for cm_node->iwdev

It is not necessary to check cm_node->iwdev in
i40iw_rem_ref_cm_node() as it can never be NULL after
a successful call out of i40iw_make_cm_node().

Signed-off-by: Chien Tin Tung <chien.tin.tung@intel.com>
Signed-off-by: Henry Orosco <henry.orosco@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>