i40iw: Avoid writing to freed memory

iwpbl->iwmr points to the structure that contains iwpbl,
which is iwmr. Setting this to NULL would result in
writing to freed memory. So just free iwmr, and return.

Fixes: d3749841 ("i40iw: add files for iwarp interface")

Reported-by: Stefan Assmann <sassmann@redhat.com>
Signed-off-by: Mustafa Ismail <mustafa.ismail@intel.com>
Signed-off-by: Shiraz Saleem <shiraz.saleem@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>