misc/sgi-xp: use NULL instead of using plain integer as pointer

This fixes the following sparse warnings:
drivers/misc/sgi-xp/xpc_main.c:210:23: warning: Using plain integer as
NULL pointer

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Reviewed-by: Steve Wahl <steve.wahl@hpe.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Link: https://lore.kernel.org/r/1615885041-68750-1-git-send-email-yang.lee@linux.alibaba.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>