dmatest: cancel thread immediately when asked for

If user have the timeout alike issues and wants to cancel the thread
immediately, the current call of wait_event_freezable_timeout is preventing to
this until timeout is expired. Thus, user will experience the unnecessary
delays.

Adding kthread_should_stop() check inside wait_event_freezable_timeout() solves
that.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>