V4L/DVB (11736): videobuf: modify return value of VIDIOC_REQBUFS ioctl

The V4L2 ioctls usually return 0 when the operation was successful
and -1 in case of error. Currently VIDIOC_REQBUFS returns the
number of buffers which is redundant because this information is
available in count field of struct v4l2_requestbuffers. The
V4L2 API specification, revision 0.24 [1] explicitly specifies for
VIDIOC_REQBUFS that the return value shall be 0 on success.

The patch was tested with v4l-test 0.13 [2] with vivi driver.

References:
[1] V4L2 API specification, revision 0.24
    http://v4l2spec.bytesex.org/spec/r13696.htm

[2] v4l-test: Test environment for Video For Linux Two API
    http://v4l-test.sourceforge.net/Signed-off-by: Márton Németh <nm127@freemail.hu>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/video/videobuf-core.c

@@ -439,6 +439,7 @@ int videobuf_reqbufs(struct videobuf_queue *q,
 	}
 
 	req->count = retval;
+	retval = 0;
 
  done:
 	mutex_unlock(&q->vb_lock);