ath9k: Fix leak in tx descriptor

ath9k: Fix leak in tx descriptor

When we reclaim the tx desc, we always assume that the
last desc is a holding desc, which is not true, and skip it.
If the tx queue is drained during channel change, internal
reset and etc, the last descriptor may not be the holding
descriptor and we fail to reclaim them. This results in the
following two issues.

1. Tx stuck - We drop all the frames coming from upper layer
due to shortage in tx desc.

2. Crash - If we fail to reclaim a tx descriptor, we miss to
update the tx BA window with the seq number of the frame
associated to that desc, which, at some point, result in
the following crash due to an assert failure in ath_tx_addto_baw().

This patch fixes these two issues.

 kernel BUG at ../drivers/net/wireless/ath/ath9k/xmit.c:180!
[155064.304164] invalid opcode: 0000 [#1] SMP
 Call Trace:
  [<fbc6d83b>] ? ath9k_tx+0xeb/0x160 [ath9k]
  [<fbbc9591>]  ipv6? __ieee80211_tx+0x41/0x120 [mac80211]
  [<fbbcb5ae>] ?  aes_i586ieee80211_master_start_xmit+0x28e/0x560 [mac80211]
  [<c037e501>]  aes_generic? _spin_lock_irqsave+0x31/0x40
  [<c02f347b>] ? dev_hard_start_xmit+0x16b/0x1c0
  [<c03058b5>] ? __qdisc_run+0x1b5/0x200
  [<fbbcda5a>] ?  af_packetieee80211_select_queue+0xa/0x100 [mac80211]
  [<c02f53b7>] ?  i915dev_queue_xmit+0x2e7/0x3f0
  [<fbbc9b49>] ? ieee80211_subif_start_xmit+0x369/0x7a0 [mac80211]
  [<c031bc35>] ? ip_output+0x55/0xb0
  [<c02e0188>] ? show_memcpy_count+0x18/0x60
  [<c02eb186>] ? __kfree_skb+0x36/0x90
  [<c02f2202>] ?  binfmt_miscdev_queue_xmit_nit+0xd2/0x110
  [<c02f347b>] ? dev_hard_start_xmit+0x16b/0x1c0
  [<c03058b5>] ? __qdisc_run+0x1b5/0x200
  [<c033bca7>] ?  scoarp_create+0x57/0x2a0
  [<c02f53b7>] ?  bridgedev_queue_xmit+0x2e7/0x3f0
  [<c03034a0>] ? eth_header+0x0/0xc0
  [<c033b95f>]  stp? arp_xmit+0x5f/0x70
  [<c033bf4f>] ? arp_send+0x5f/0x70
  [<c033c8f5>]  bnep? arp_solicit+0x105/0x210
  [<c02fa5aa>] ? neigh_timer_handler+0x19a/0x390
  [<c013bf88>] ? run_timer_softirq+0x138/0x210
  [<c02fa410>] ?  ppdevneigh_timer_handler+0x0/0x390
  [<c02fa410>] ? neigh_timer_handler+0x0/0x390

Signed-off-by: Vasanthakumar Thiagarajan <vasanth@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>