hwmon: (f75375s) Fix value range for PWM modes

Accepted value range for PWM modes was 0..4, even though only 0..3 is
subsequently used. Limit permitted value range to 0..3 to avoid unpredictable
behavior.

Cc: Riku Voipio <riku.voipio@iki.fi>
Reviewed-by: Bjoern Gerhart <oss@extracloud.de>
Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>