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

New warning since commit ab88ca488b8af66c3defa165874e81e695319a19,
"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: Stefan Richter <stefanr@s5r6.in-berlin.de>