unisys: use kthread_should_stop in the thread

convert the users of should_stop variable into kthread_should_stop() API.

Cc: Ken Cox <jkc@redhat.com>
Cc: Benjamin Romer <benjamin.romer@unisys.com>
Signed-off-by: Devendra Naga <devendra.aaru@gmail.com>
Signed-off-by: Benjamin Romer <benjamin.romer@unisys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>