cpufreq: interactive: Put Sanity check to fix possible issue

    It prevents rarely observed crash while changing governors through sysfs.

    [   61.429271] Unable to handle kernel NULL pointer dereference at virtual a
    [   61.429612] pgd = c0004000
    [   61.429732] [00000008] *pgd=00000000
    [   61.429919] Internal error: Oops: 5 [#1] PREEMPT SMP ARM
    [   61.430625] CPU: 0 PID: 720 Comm: thread_hotplug_ Not tainted 3.10.9 #1
    [   61.437202] task: cf9589c0 ti: ce1b8000 task.ti: ce1b8000
    [   61.442636] PC is at kthread_stop+0x14/0xdc
    [   61.446773] LR is at cpufreq_governor_interactive+0x334/0x670

Change-Id: Icca866c2ebbf58086900fac5cf40b1b2c6cb46b7
Signed-off-by: Yadwinder Singh Brar <yadi.brar@samsung.com>
Reviewed-on: http://107.108.3.115:8081/5375


Reviewed-by: Pankaj Kumar Dubey <pankaj.dubey@samsung.com>
Reviewed-by: ritesh solanki <r.solanki@samsung.com>
Tested-by: ritesh solanki <r.solanki@samsung.com>