When a wdev changes network namespace, its wdev ID will get
reassigned since NETDEV_REGISTER is called again, in the new
network namespace. Avoid that by checking if it was already
assigned before, and document why we do that.

Reported-and-tested-by: Arend Van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>