hwmon: (corsair-psu) fix plain integer used as NULL pointer

sparse warnings: (new ones prefixed by >>)
>> drivers/hwmon/corsair-psu.c:536:82: sparse: sparse: Using plain
   integer as NULL pointer

Fixes: d115b51e

 ("hwmon: add Corsair PSU HID controller driver")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Wilken Gottwalt <wilken.gottwalt@posteo.net>
Link: https://lore.kernel.org/r/YY9hAL8MZEQYLYPf@monster.localdomain


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