device: don't use platform singleton getter in device subclasses
Reduce the use of NM_PLATFORM_GET / nm_platform_get() to get the platform singleton instance. For one, this is a step towards supporting namespaces, where we need to use different NMNetns/NMPlatform instances depending on in which namespace the device lives. Also, we should reduce our use of singletons. They are difficult to coordinate on shutdown. Instead there should be a clear order of dependencies, expressed by owning a reference to those singelton instances. We already own a reference to the platform singelton, so use it and avoid NM_PLATFORM_GET.
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