sata_dwc_460ex: fix return value of dma_dwc_xfer_setup()

The caller expects this function to return the DMA channel number on success,
while it returns 0...

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>