rtl8xxxu: Fix memory leak in handling rxdesc16 packets

A device running without RX package aggregation could return more data
in the USB packet than the actual network packet. In this case the
could would clone the skb but then determine that that there was no
packet to handle and exit without freeing the cloned skb first.

This has so far only been observed with 8188eu devices, but could
affect others.

Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>