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 <[e-mail]>
Reviewed-by: Steve Wahl <[e-mail]>
Signed-off-by: Yang Li <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/misc/sgi-xp/xpc_main.c

···
 {
 	xpc_arch_ops.heartbeat_init();
 	timer_setup(&xpc_hb_timer, xpc_hb_beater, 0);
-	xpc_hb_beater(0);
+	xpc_hb_beater(NULL);
 }
 
 static void