wifi: mt76: do not run mt76_unregister_device() on unregistered hw

In the Linux kernel, the following vulnerability has been resolved: wifi: mt76: do not run mt76_unregister_device() on unregistered hw Trying to probe a mt7921e pci card without firmware results in a successful probe where ieee80211_register_hw hasn't been called. When removing the driver, ieee802111_unregister_hw is called unconditionally leading to a kernel NULL pointer dereference. Fix the issue running mt76_unregister_device routine just for registered hw.