usb: musb: host: make musb_tx_dma_set_mode_*() *void*

Now that the DMA engine check was moved to musb_tx_dma_porgram(), both
musb_tx_dma_set_mode_cppi_tusb() and musb_tx_dma_set_mode_mentor() always
return 0, so we can  make both these functions *void*.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
[b-liu@ti.com: revise subject prefix]
Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>