usb: f_qc_rndis: set context for encapsulated response

Setting the req->context field for encapsulated responses
as well as for encapsulated commands. This prevents a
corner case where the request is initially used for
a response, prior to using it for a command.

Change-Id: I466bf9c02669161df3565d3f6f9a064aabffa822
Signed-off-by: Dov Levenglick <dovl@codeaurora.org>