hwmon: (mlxreg-fan) Return zero speed for broken fan

Currently for broken fan driver returns value calculated based on error
code (0xFF) in related fan speed register.
Thus, for such fan user gets fan{n}_fault to 1 and fan{n}_input with
misleading value.

Add check for fan fault prior return speed value and return zero if
fault is detected.

Fixes: 65afb4c8e7 ("hwmon: (mlxreg-fan) Add support for Mellanox FAN driver")
Signed-off-by: Vadim Pasternak <vadimp@nvidia.com>
Link: https://lore.kernel.org/r/20230212145730.24247-1-vadimp@nvidia.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

drivers/hwmon/mlxreg-fan.c

@@ -155,6 +155,12 @@ mlxreg_fan_read(struct device *dev, enum hwmon_sensor_types type, u32 attr,
 			if (err)
 				return err;
 
+			if (MLXREG_FAN_GET_FAULT(regval, tacho->mask)) {
+				/* FAN is broken - return zero for FAN speed. */
+				*val = 0;
+				return 0;
+			}
+
 			*val = MLXREG_FAN_GET_RPM(regval, fan->divider,
 						  fan->samples);
 			break;