firewire: ohci: 0 may be a valid DMA address

I was told that there are obscure architectures with non-coherent DMA
which may DMA-map to bus address 0.  We shall not use 0 as a magic
number of uninitialized bus address variables.

The packet->payload_length > 0 test cannot be used either (except in
at_context_queue_packet) because local requests are not DMA-mapped
regardless of payload_length.  Hence add a state flag to struct
fw_packet.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>

drivers/firewire/core-transaction.c

@@ -226,7 +226,7 @@ static void fw_fill_request(struct fw_packet *packet, int tcode, int tlabel,
 	packet->speed = speed;
 	packet->generation = generation;
 	packet->ack = 0;
-	packet->payload_bus = 0;
+	packet->payload_mapped = false;
 }
 
 /**
@@ -601,7 +601,7 @@ void fw_fill_response(struct fw_packet *response, u32 *request_header,
 		WARN(1, KERN_ERR "wrong tcode %d", tcode);
 	}
 
-	response->payload_bus = 0;
+	response->payload_mapped = false;
 }
 EXPORT_SYMBOL(fw_fill_response);