hwmon: (asus-ec-sensors) Add checks for devm_kcalloc

As the devm_kcalloc may return NULL, the return value needs to be checked
to avoid NULL poineter dereference.

Fixes: d0ddfd24

 ("hwmon: (asus-ec-sensors) add driver for ASUS EC")
Signed-off-by: Yuan Can <yuancan@huawei.com>
Link: https://lore.kernel.org/r/20221125014329.121560-1-yuancan@huawei.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>