USB: u_serial: Don't allow UDC to append and send a zero length packet

Two zero length packets are sent by the target in the current scenario:
1. From the device controller driver (as the req->zero flag is set if
   it is the last packet).
2. From the u_serial driver when the length of the packet is zero while
   checking for the next packet in the list.

Hence removing the code for the case1, as the zero length packet is
sent via u_serial function driver.

Change-Id: Ifc1bc9cf8435149534c271343ea7ef64b9ed4d4a
CRs-Fixed: 350179
Signed-off-by: Rajkumar Raghupathy <raghup@codeaurora.org>