firewire: fw-ohci: shut up a superfluous compiler warning

New warning since commit ab88ca48,
"firewire: fw-ohci: missing dma_unmap_single":
drivers/firewire/fw-ohci.c: In function 'at_context_transmit':
drivers/firewire/fw-ohci.c:609: warning: 'payload_bus' may be used
 uninitialized in this function

Access to payload_bus is conditional on packet->payload_length > 0,
and that won't change while in at_context_queue_packet.
Signed-off-by: NStefan Richter <stefanr@s5r6.in-berlin.de>

drivers/firewire/fw-ohci.c

@@ -606,7 +606,7 @@ static int
 at_context_queue_packet(struct context *ctx, struct fw_packet *packet)
 {
 	struct fw_ohci *ohci = ctx->ohci;
-	dma_addr_t d_bus, payload_bus;
+	dma_addr_t d_bus, uninitialized_var(payload_bus);
 	struct driver_data *driver_data;
 	struct descriptor *d, *last;
 	__le32 *header;