usb: musb: tusb6010: Clean up tusb_omap_dma structure

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