usb: gadget: u_ether: add scatter/gather support

scatter gather allows data aggregation or header addition without
memcpy. This will greatly reduce cpu mips. Also add process all
ingress skbs in workqueue so that more packets can be aggregated.

Change-Id: I60439173160c2121367d0eca117f9bbdeba0336d
Signed-off-by: Vamsi Krishna <vskrishn@codeaurora.org>