usb: musb: cppi41: Enable in device-TX mode

Since the musb-gadget code now calls the dma engine properly it is
possible to enable it for the TX path in device mode.
AM335x Advisory 1.0.13 says that we may lose the toggle bit on multiple
RX transfers. There is a workaround in host mode but none in device mode
and therefore RX transfers are disabled.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>