unisys: use simpler kthread_ API

The code does the checks on should_stop variable in the kernel
threads. The uisthread_stop function sets the should_stop and calls
KILL (eventually kill_pid) to stop the thread.

The checking of should_stop variable can be replaced to a call to
kthread_should_stop function and the setting of the should_stop and
a call to KILL can be replaced with kthread_stop function.

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>