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>

drivers/misc/sgi-xp/xpc_main.c

@@ -207,7 +207,7 @@ xpc_start_hb_beater(void)
 {
 	xpc_arch_ops.heartbeat_init();
 	timer_setup(&xpc_hb_timer, xpc_hb_beater, 0);
-	xpc_hb_beater(0);
+	xpc_hb_beater(NULL);
 }
 
 static void