habanalabs: use NULL to initialize array of pointers

This patch fixes the following sparse warnings:

drivers/misc/habanalabs/hwmon.c:20:56: warning: Using plain integer as NULL pointer

Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>