usb: musb: ux500: take the dma_mask from coherent_dma_mask

The dma_mask will always be the same as the coherent_dma_mask, so let's
cut down on the platform_data burden and set it as such in the driver.
This also saves us from supporting it separately when we come to enable
this driver for Device Tree.

Cc: linux-usb@vger.kernel.org
Acked-by: Felipe Balbi <balbi@ti.com>
Acked-by: Fabio Baltieri <fabio.baltieri@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>