wifi: brcmfmac: Fix potential use-after-free issue when stopping watchdog task

In the Linux kernel, the following vulnerability has been resolved: wifi: brcmfmac: Fix potential use-after-free issue when stopping watchdog task Watchdog task might end between send_sig() and kthread_stop() calls, what results in the use-after-free issue. Fix this by increasing watchdog task reference count before calling send_sig() and dropping it by switching to kthread_stop_put().