Fix a bug where bf->bf_skb needs to be set to NULL because the skbuff have already...

Fix a bug where bf->bf_skb needs to be set to NULL because the skbuff have already been freed by
ath_tx_capture, for each frame in the list we have already freed the skb but the original pointer to
the list of skbs is still in bf->bf_skb.  So when bf is returned with ath_return_txbuf it tries to
double-free, resuilting a spew of backtraces/warnings when reference debugging is enabled.

Not to mention the fact that the skb is double-freed.

git-svn-id: http://madwifi-project.org/svn/madwifi/trunk@3220 0192ed92-7a03-0410-a25b-9323aeb14dbd