thermal/drivers/cpufreq_cooling: Fix slab OOB issue

Slab OOB issue is scanned by KASAN in cpu_power_to_freq().
If power is limited below the power of OPP0 in EM table,
it will cause slab out-of-bound issue with negative array
index.

Return the lowest frequency if limited power cannot found
a suitable OPP in EM table to fix this issue.

Backtrace:
[<ffffffd02d2a37f0>] die+0x104/0x5ac
[<ffffffd02d2a5630>] bug_handler+0x64/0xd0
[<ffffffd02d288ce4>] brk_handler+0x160/0x258
[<ffffffd02d281e5c>] do_debug_exception+0x248/0x3f0
[<ffffffd02d284488>] el1_dbg+0x14/0xbc
[<ffffffd02d75d1d4>] __kasan_report+0x1dc/0x1e0
[<ffffffd02d75c2e0>] kasan_report+0x10/0x20
[<ffffffd02d75def8>] __asan_report_load8_noabort+0x18/0x28
[<ffffffd02e6fce5c>] cpufreq_power2state+0x180/0x43c
[<ffffffd02e6ead80>] power_actor_set_power+0x114/0x1d4
[<ffffffd02e6fac24>] allocate_power+0xaec/0xde0
[<ffffffd02e6f9f80>] power_allocator_throttle+0x3ec/0x5a4
[<ffffffd02e6ea888>] handle_thermal_trip+0x160/0x294
[<ffffffd02e6edd08>] thermal_zone_device_check+0xe4/0x154
[<ffffffd02d351cb4>] process_one_work+0x5e4/0xe28
[<ffffffd02d352f44>] worker_thread+0xa4c/0xfac
[<ffffffd02d360124>] kthread+0x33c/0x358
[<ffffffd02d289940>] ret_from_fork+0xc/0x18

Fixes: 371a3bc7 ("thermal/drivers/cpufreq_cooling: Fix wrong frequency converted from power")
Signed-off-by: brian-sy yang <brian-sy.yang@mediatek.com>
Signed-off-by: Michael Kao <michael.kao@mediatek.com>
Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>
Cc: stable@vger.kernel.org #v5.7
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20201229050831.19493-1-michael.kao@mediatek.com

drivers/thermal/cpufreq_cooling.c

@@ -116,7 +116,7 @@ static u32 cpu_power_to_freq(struct cpufreq_cooling_device *cpufreq_cdev,
 {
 	int i;
 
-	for (i = cpufreq_cdev->max_level; i >= 0; i--) {
+	for (i = cpufreq_cdev->max_level; i > 0; i--) {
 		if (power >= cpufreq_cdev->em->table[i].power)
 			break;
 	}