usb: device_next: fix CDC NCM datagram pointers processing

The implementation should not assume that NDP is placed immediately
after NTH. According to the specification (revision 1.0), NDP can be
anywhere in the transfer buffer. There is also always a "terminating
zero datagram pointer entry", which we should also use to terminate the
datagram processing loop.

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>