usb: musb: cppi_dma: Clean up cppi structure

A pointer to musb is now present in the dma_controller structure.
Remove the one present in cppi structure.

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