usb: check RNDIS message length

When processing remote NDIS control message packets, the USB Net
device emulator uses a fixed length(4096) data buffer. The incoming
packet length could exceed this limit. Add a check to avoid it.

Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
Message-id: 1455648821-17340-2-git-send-email-ppandit@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
