HSD #14015013554: firmware: stratix10-svc: Fixed thread stop

commit 2dedb8412f0428425cef653cd784a351273e4efd from
https://github.com/altera-opensource/linux-socfpga.git

 socfpga-5.15.30-lts.

Ensures that the thread task is checked for validity before calling kthread_stop
when the user aborts.

Signed-off-by: Ang Tien Sung <tien.sung.ang@intel.com>
Signed-off-by: Liwei Song <liwei.song@windriver.com>