net: usb: Do not modify usbnet device context upon rx complete

Upon rx completion, the driver parses the mux header to identify
the actual net device the IP packet is intended for. At this point
the driver also modifies the usbnet device context to a new usbnet
device, which is derived from the associated IP mux header. This
causes the rx skb queue list corruption when skb is moved
incorrectly from new usbnet device's rx skb queue. Hence do not
overwrite the original usbnet device instance which is required
to move skb from rx skb queue to done queue.

CRs-Fixed: 598321
Change-Id: I642f877dd2f758ddbb868de986dd223bad10eee9
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>