usb: musb: musb_cppi41: Defer probe only if DMA is not ready

If dma_request_slave_channel() failed to return a channel,
then the driver will print an error and request to defer probe,
regardless of the cause of the failure.
Defer if the DMA is not ready yet otherwise print an error.

Signed-off-by: Alexandre Bailon <abailon@baylibre.com>
Reviewed-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>