wifi: mt76: mt7915: fix use-after-free bugs in mt7915_mac_dump_work()

wifi: mt76: mt7915: fix use-after-free bugs in mt7915_mac_dump_work()

When the mt7915 pci chip is detaching, the mt7915_crash_data is
released in mt7915_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
mt7915_mac_dump_work().

The race condition can occur as follows:

CPU 0 (removal path)               | CPU 1 (workqueue)
mt7915_pci_remove()                | mt7915_sys_recovery_set()
 mt7915_unregister_device()        |  mt7915_reset()
  mt7915_coredump_unregister()     |   queue_work()
   vfree(dev->coredump.crash_data) | mt7915_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
mt7915_unregister_device() to synchronize with any pending
or executing dump work.

Fixes: 4dbcb9125cc3 ("wifi: mt76: mt7915: enable coredump support")
Signed-off-by: Duoming Zhou <duoming@zju.edu.cn>
Link: https://patch.msgid.link/20260130145759.84272-1-duoming@zju.edu.cn
Signed-off-by: Felix Fietkau <nbd@nbd.name>