usb: f_gsi: Fix rndis message parsing of erroneous requests

In the completion 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: I52001128ac421e58e1801eebc243a8c91618582c
Signed-off-by: Sriharsha Allenki <sallenki@codeaurora.org>