ath9k_htc: rare leak in ath9k_hif_usb_alloc_tx_urbs()

This is obviously a small picky thing.  The original error handling code
doesn't free the most recent allocations which haven't been added to the
hif_dev->tx.tx_buf list yet.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Sujith <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>