hwmon: (max6650) Fix unused variable warning

The newly added variable is only used in an #if block:

drivers/hwmon/max6650.c: In function 'max6650_probe':
drivers/hwmon/max6650.c:766:33: error: unused variable 'cooling_dev' [-Werror=unused-variable]

Change the #if to if() so the compiler can see what is actually
going on.

Fixes: a8463754a5a9 ("hwmon: (max6650) Use devm function to register thermal device")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>