i40iw: Receive notification events correctly

Device notifications are not received after the first interface is
closed; since there is an unregister for notifications on every
interface close. Correct this by unregistering for device
notifications only when the last interface is closed. Also, make
all operations on the i40iw_notifiers_registered atomic as it
can be read/modified concurrently.

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

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>