ath5k: rename ath5k_txbuf_free() to ath5k_txbuf_free_skb()

Rename ath5k_txbuf_free() to ath5k_txbuf_free_skb() since this is what it does:
it frees the skb and not the buf. Same for ath5k_rxbuf_free().

Signed-off-by: Bruno Randolf <br1@einfach.org>
Acked-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>