hwmon: (max31790) revise the scale to write pwm

Since the value for PWMOUT Target Duty Cycle register is a 9 bit
left-justified value that ranges from 0 to 511 and is contained in 2
bytes.

There is an issue that the PWM signal recorded by oscilloscope would
not be on consistently if we set PWM to 100% to the driver.

It is because the LSB of the 9 bit would always be zero if it just
left shift 8 bit for the value that write to PWMOUT Target Duty
Cycle register.

Therefore, revise the scale of the value that was written to pwm input
from 255 to 511 and modify the value to left-justified value.

Signed-off-by: Delphine CC Chiu <Delphine_CC_Chiu@wiwynn.com>
Link: https://lore.kernel.org/r/20240416022211.859483-1-Delphine_CC_Chiu@wiwynn.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

drivers/hwmon/max31790.c

@@ -49,6 +49,9 @@
 
 #define NR_CHANNEL			6
 
+#define PWM_INPUT_SCALE	255
+#define MAX31790_REG_PWMOUT_SCALE	511
+
 /*
  * Client data (each client gets its own)
  */
@@ -343,10 +346,13 @@ static int max31790_write_pwm(struct device *dev, u32 attr, int channel,
 			err = -EINVAL;
 			break;
 		}
+
+		val = DIV_ROUND_CLOSEST(val * MAX31790_REG_PWMOUT_SCALE,
+					PWM_INPUT_SCALE);
 		data->valid = false;
 		err = i2c_smbus_write_word_swapped(client,
 						   MAX31790_REG_PWMOUT(channel),
-						   val << 8);
+						   val << 7);
 		break;
 	case hwmon_pwm_enable:
 		fan_config = data->fan_config[channel];