usb: f_qc_rndis: Fix rndis message parsing of erroneous requests

In the competion handler of 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: I15ffd0bef4b42adf2300085dc3720d599e647cb5
Signed-off-by: Sriharsha Allenki <sallenki@codeaurora.org>