USB: u_ether: Set complete handler before queueing to endpoint

rndis function driver will queue requests for TX endpoints in
tasklet context and interrupt context. While queueing requests
in interrupt context, not setting completion handler before
queueing to TX endpoint. Due to this, there is a chance that
usb request will be queued to USB HW without setting completion
handler. This results in NULL pointer dereference when that
particular usb request is completed. Hence mark complete handler
before queueing usb request in interrupt context.

Change-Id: I4cf235ace13f05f1e7bf2415b8a7dccf09d1115d
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>
Signed-off-by: Tarun Gupta <tarung@codeaurora.org>
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>