usb: ci_udc: set ep->req.actual after transfer

At least drivers/usb/gadget/storage_common.c expects that ep->req.actual
contain the number of bytes actually transferred. (At least in practice,
I observed it failing to work correctly unless this was the case).

However, ci_udc.c modifies ep->req.length instead. I assume that .length
 is supposed to represent the allocated buffer size, whereas .actual is
supposed to represent the actual number of bytes transferred. In the OUT
transaction case, this may happen simply because the host sends a smaller
 packet than the max possible size, which is quite legal. In the IN case,
transferring fewer bytes than requested could presumably happen as an
error.

Modify handle_ep_complete() to write to .actual rather than modifying
.length.

Signed-off-by: Stephen Warren <swarren@nvidia.com>