staging: r8712u: Fix leak of skb

There are two types of messages queued for RX. The major type, which does
I/O on the device, was being handled properly. The skbs that communicated
with the firmware were being leaked.

While rewriting the code that sets up the skb, it was possible to remove
the private variable indicating that the old skb could be reused.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>