hwmon: (nct6775) Use return value from find_temp_source

hwmon: (nct6775) Use return value from find_temp_source

smatch complains that we don't use the return value from find_temp_source().
Valid point, only find_temp_source() doesn't return a valid error code.
Have it return a valid error code and use it.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>