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

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

Origin: https://patchwork.kernel.org/project/linux-wireless/patch/be3457d82f4e44bb71a22b2b5db27b644a37b1e1.1677107277.git.lorenzo@kernel.org/
Bug-Debian: https://bugs.debian.org/1029116

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.

Link: https://bugs.debian.org/1029116
Link: https://bugs.kali.org/view.php?id=8140
Reported-by: Stuart Hayhurst <stuart.a.hayhurst@gmail.com>
Fixes: 1c71e03afe4b ("mt76: mt7921: move mt7921_init_hw in a dedicated work")
Tested-by: Helmut Grohne <helmut@freexian.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Currently queued for review for the v6.3 release cycle, backported in
advance to prevent users from getting stuck in the installer.

Signed-off-by: Cyril Brulebois <kibi@debian.org>
Gbp-Pq: Topic bugfix/all
Gbp-Pq: Name wifi-mt76-do-not-run-mt76_unregister_device-on-unregistered-hw.patch