hwmon: (mlxreg-fan) Modify PWM connectivity validation

Validate PWM connectivity only for additional PWM - "pwm1" is connected
on all systems, while "pwm2" - "pwm4" are optional. Validate
connectivity only for optional attributes by reading of related "pwm{n}"
registers - in case "pwm{n}" is not connected, register value is
supposed to be 0xff.

Signed-off-by: Vadim Pasternak <vadimp@nvidia.com>
Link: https://lore.kernel.org/r/20210926053541.1806937-2-vadimp@nvidia.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

drivers/hwmon/mlxreg-fan.c

@@ -488,9 +488,14 @@ static int mlxreg_fan_config(struct mlxreg_fan *fan,
 				return -EINVAL;
 			}
 
-			err = mlxreg_pwm_connect_verify(fan, data);
-			if (err)
-				return err;
+			/* Validate if more then one PWM is connected. */
+			if (pwm_num) {
+				err = mlxreg_pwm_connect_verify(fan, data);
+				if (err < 0)
+					return err;
+				else if (!err)
+					continue;
+			}
 
 			fan->pwm[pwm_num].reg = data->reg;
 			fan->pwm[pwm_num].connected = true;