usb: musb: ux500: Add check for NULL board data

Dan Carpenter's automatic Smatch checker found an anomaly in the ux500
MUSB driver, whereby board data was checked before use in all but one
occasion. It is believed that it needs to be checked every time.

Smatch complaint:
  drivers/usb/musb/ux500_dma.c:335 ux500_dma_controller_start()
         error: we previously assumed 'data' could be null (see line 313)

Cc: Felipe Balbi <balbi@ti.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-usb@vger.kernel.org
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>