hwmon: (it87) Do not overwrite bit 2..6 of pwm control registers

hwmon: (it87) Do not overwrite bit 2..6 of pwm control registers

In IT8620E, after setting pwm control to manual, it was observed that
pwm values for fan 4..6 have reversed results (writing 0 results in fans
running at full speed, writing 255 results in fans turned off).

With the new PWM control, pwm polarity for pwm control 4..6 is specified
in its pwm control registers. Those registers are overwritten when setting
the pwm mode or the temperature mapping. Do not touch bit 2..6 of pwm
control registers on register writes to fix the problem.

Cc: stable@vger.kernel.org # 4.9+
Signed-off-by: Guenter Roeck <linux@roeck-us.net>