usb: musb: ux500: set dma config for both src and dst

The dma driver requires both src and dst to be set.
This fix is needed in order to run gadget mass storage.
Patch is verified on snowball.

Signed-off-by: Per Forlin <per.forlin@linaro.org>
Acked-by: Mian Yousaf Kaukab <mian.yousaf.kaukab@stericsson.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>