cpufreq: interactive: add loop condition for speedchange_task

Currently, the speedchange_task is checked should_stop state
after called schedule(). In this case, the speedchange_task
can not exit when called kthread_stop with TASK_RUNNING. Because
the speedchange_task entered sleep state with TASK_INTERRUTIBLE.
So, add the should_stop state check to while loop condition.

Change-Id: I5e48bb5be6b5af3cea955b3304eee1474b67ae52
Signed-off-by: Taekki Kim <taekki.kim@samsung.com>