wifi: mt76: mt7996: fix use-after-free bugs in mt7996_mac_dump_work()

wifi: mt76: mt7996: fix use-after-free bugs in mt7996_mac_dump_work()

When the mt7996 pci chip is detaching, the mt7996_crash_data is
released in mt7996_coredump_unregister(). However, the work item
dump_work may still be running or pending, leading to UAF bugs
when the already freed crash_data is dereferenced again in
mt7996_mac_dump_work().

The race condition can occur as follows:

CPU 0 (removal path)               | CPU 1 (workqueue)
mt7996_pci_remove()                | mt7996_sys_recovery_set()
 mt7996_unregister_device()        |  mt7996_reset()
  mt7996_coredump_unregister()     |   queue_work()
   vfree(dev->coredump.crash_data) | mt7996_mac_dump_work()
                                   |  crash_data-> // UAF

Fix this by ensuring dump_work is properly canceled before
the crash_data is deallocated. Add cancel_work_sync() in
mt7996_unregister_device() to synchronize with any pending
or executing dump work.

Fixes: 878161d5d4a4 ("wifi: mt76: mt7996: enable coredump support")
Signed-off-by: Duoming Zhou <duoming@zju.edu.cn>
Link: https://patch.msgid.link/20260131024731.18741-1-duoming@zju.edu.cn
Signed-off-by: Felix Fietkau <nbd@nbd.name>