usb: musb: cppi_dma: export cppi_interrupt()

Now that DaVinci glue layer can be modular, we must export cppi_interrupt()
that it may call...

Cc: stable@vger.kernel.org # 3.0+
Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>