usb: gadget: f_cdev: Send zero length packets in modem data calls

The USB protocol requires zero length packets to be sent when the data
transmission ends on a USB packet size boundary. This fix enables sending
zero length packets when required.

Change-Id: I0269ec7ff25c82000f2a5bc4adb449bc0231c66c
Signed-off-by: Pratham Pratap <prathampratap@codeaurora.org>