hwmon: (adm1031) Fix compiler warning

Some configurations produce the following compile warning:

drivers/hwmon/adm1031.c: In function 'set_fan_auto_channel':
drivers/hwmon/adm1031.c:292: warning: 'reg' may be used uninitialized in this function

While this is a false positive, it can easily be fixed by overloading the return
value from get_fan_auto_nearest with both register value and error return code
(the register value is never negative). Coincidentially, that also reduces
module size by a few bytes.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Jean Delvare <khali@linux-fr.org>