staging: r8712u: Do not queue cloned skb

Some post-3.4 kernels have a problem when a cloned skb is used in the
RX path. This patch handles one such case for r8712u.

The patch was suggested by Eric Dumazet.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Stable <stable@vger.kernel.org>  [v3.4+]
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>