hwmon: (asus_atk0110) fix uninitialized data access

The latest gcc-7 snapshot adds a warning to point out that when
atk_read_value_old or atk_read_value_new fails, we copy
uninitialized data into sensor->cached_value:

drivers/hwmon/asus_atk0110.c: In function 'atk_input_show':
drivers/hwmon/asus_atk0110.c:651:26: error: 'value' may be used uninitialized in this function [-Werror=maybe-uninitialized]

Adding an error check avoids this. All versions of the driver
are affected.

Fixes: 2c03d07a ("hwmon: Add Asus ATK0110 support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Luca Tettamanti <kronos.it@gmail.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>