usb: f_rndis: Fix rndis message parsing of erroneous requests

In the completion handler of the rndis command requests we
are parsing the request buffers without checking the
status of the request. This might cause parsing of the
erroneous requests. Fix this by checking the status
of the request before parsing the request buffer.

Change-Id: I476c6c82d367f6f5fc6eff25b049b3323b68b859
Signed-off-by: Sriharsha Allenki <sallenki@codeaurora.org>