i40iw: Protect req_resource_num update

In i40iw_alloc_resource(), ensure that the update to
req_resource_num is protected by the lock.

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>