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

Fixes: 8e06af71 ("i40iw: add main, hdr, status")
Reported-by: NStefan Assmann <sassmann@redhat.com>
Signed-off-by: NMustafa Ismail <mustafa.ismail@intel.com>
Signed-off-by: NShiraz Saleem <shiraz.saleem@intel.com>
Signed-off-by: NDoug Ledford <dledford@redhat.com>