hwmon: (w83l768ng) Fix fan speed control range

The W83L786NG stores the fan speed on 4 bits while the sysfs interface
uses a 0-255 range. Thus the driver should scale the user input down
to map it to the device range, and scale up the value read from the
device before presenting it to the user. The reserved register nibble
should be left unchanged.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: stable@vger.kernel.org
Reviewed-by: Guenter Roeck <linux@roeck-us.net>