usb: gadget: Improve downlink throughput by enabling aggregation

There is low throughput of around 100Mbps for CAT6 seen on UDP-DL for
rmnet when using the u_ether transport.
Rmnet driver processes 10 USB requests at a time and then relinquishes
the CPU. Due to this implementation, network queue is being flow
controlled very often and also flow control is disabled after a
long time. Hence DL throughput decreases.
So aggregate 10 IP packets at a time and send in one USB request,
which improves the throughput from 100Mbps to 294Mbps.

CRs-fixed: 699603
Change-Id: I71c7f72c7c10a25bdcc50fc3d6859bff9555a500
Signed-off-by: Azhar Shaikh <azhars@codeaurora.org>