staging: vchiq_arm: Fix possible NPR of keep-alive thread

staging: vchiq_arm: Fix possible NPR of keep-alive thread

In case vchiq_platform_conn_state_changed() is never called or fails before
driver removal, ka_thread won't be a valid pointer to a task_struct. So
do the necessary checks before calling kthread_stop to avoid a crash.

Fixes: 863a756aaf49 ("staging: vc04_services: vchiq_core: Stop kthreads on vchiq module unload")
Signed-off-by: Stefan Wahren <wahrenst@gmx.net>
Link: https://lore.kernel.org/r/20250309125014.37166-3-wahrenst@gmx.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>