mediatek: fix pwn fan settings for sinovoip bpi-r3

mediatek: fix pwn fan settings for sinovoip bpi-r3

Popular bpi-r3 pwm fans like this one

  https://www.amazon.com/youyeetoo-Barebone-Fan-BPI-R3-Integrated/dp/B0CCCTY8PS

will not work properly with current openwrt-23.05/24.10 firmware.
Trying different pwm setting

  echo $value > /sys/devices/platform/pwm-fan/hwmon/hwmon1/pwm1

I found:

  pwm1 value     fan rotation speed   cpu temperature     notes
  -----------------------------------------------------------------
    0            maximal              31.5 Celsius        too noisy
   40            optimal              35.2 Celsius        no noise hearable
   95            minimal
   above 95      does not rotate      55.5 Celsius
  -----------------------------------------------------------------

At the moment we have following cooling levels:

  cooling-levels = <255 96 0>;

for cpu-active-high, cpu-active-medium and cpu-active-low modes correspondingly.
Thus only cpu-active-high and cpu-active-low are usable. I think this is wrong.

This patch fixes cpu-active-medium settings for bpi-r3 board.

PS: I know, the patch is not ideal as it can break pwm fan for some users.
    There are some peoples that use handmade cooling solutions, but:
      * discussed cooler is the only 'official' pwm cooler for bpi-r3
        available on the market.
      * most peoples will use passive cooling available on the market or
        the discussed cooler.
      * the pwm-fan dts section was added before the official cooler
        appears on the market.
    Thus it should not be a lot of harm from this fix.

Signed-off-by: Mikhail Kshevetskiy <mikhail.kshevetskiy@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/16974
Signed-off-by: Robert Marko <robimarko@gmail.com>