hwmon: (it87) Support disabling fan control for all pwm control and chips

On/Off mode is only supported for pwm controls 0-2, and not supported at all for
IT8603E/IT8623E. For pwm controls 3-6 and for IT8603E/IT8623E, SmartGuardian mode
is always enabled. Use it and set the pwm value to the maximum if fan control
is disabled.

Tested-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>