USB: gadget: rndis: Optimize tx path for better performance

Current rndis header addition and aggregation steps:
1. Increase skb headroom by size of rndis header.
2. Copy skb data with newly created skb having more headroom.
3. Create and add rndis header with it.
4. Again copy skb data with usb request buffer for aggregation purpose.

New optimized steps :
1. Create and copy rndis header directly with usb request buffer
2. Copy skb data with usb request buffer

Above new steps avoids cpu intensive skb operations which reduces CPU
usage and provide better throughput.

This change also assigns completion handle with usb request at the time
of usb request creation instead of assigning everytime before queueing
usb request. Also create debugfs for ether stats.

Change-Id: Ifc64d6a8573f6a24e28c73ef9a19b62649e171cd
Signed-off-by: Mayank Rana <mrana@codeaurora.org>
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>