i40iw: Add missing check for interface already open

In i40iw_open(), check if interface is already open
and return success if it is.

Fixes: 8e06af71 ("i40iw: add main, hdr, status")

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>