hwmon/smsc47b397: Don't report missing fans as spinning at 82 RPM

Also protects ourselves against a possible division by zero.
Signed-off-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Mark M. Hoffman <mhoffman@lightlink.com>

drivers/hwmon/smsc47b397.c

@@ -174,6 +174,8 @@ static SENSOR_DEVICE_ATTR(temp4_input, S_IRUGO, show_temp, NULL, 3);
    REG: count of 90kHz pulses / revolution */
 static int fan_from_reg(u16 reg)
 {
+	if (reg == 0 || reg == 0xffff)
+		return 0;
 	return 90000 * 60 / reg;
 }