i40iw: Do not set self-referencing pointer to NULL after kfree

In i40iw_free_virt_mem(), do not set mem->va to NULL
after freeing it as mem->va is a self-referencing pointer
to mem.

Fixes: 4e9042e6 ("i40iw: add hw and utils files")

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>