hwmon: (f75375s) Fix writes to the pwm* attribute for the F75387

For the F75387, the register holding the current PWM duty cycle value is
r/o; changing it requires writing to the fan expect register instead.

Signed-off-by: Nikolaus Schulz <mail@microschulz.de>
[guenter.roeck@ericsson.com: Simplified function parameters]
Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>