iwlwifi: fix the Transmit Frame Descriptor rings

iwlwifi: fix the Transmit Frame Descriptor rings

The logic that allows to have a short TFD queue was completely wrong.
We do maintain 256 Transmit Frame Descriptors, but they point to
recycled buffers. We used to attach and de-attach different TFDs for
the same buffer and it worked since they pointed to the same buffer.

Also zero the number of BDs after unmapping a TFD. This seems not
necessary since we don't reclaim the same TFD twice, but I like
housekeeping.

This patch solves this warning:

[ 6427.079855] WARNING: at lib/dma-debug.c:866 check_unmap+0x727/0x7a0()
[ 6427.079859] Hardware name: Latitude E6410
[ 6427.079865] iwlwifi 0000:02:00.0: DMA-API: device driver tries to free DMA memory it has not allocated [device address=0x00000000296d393c] [size=8 bytes]
[ 6427.079870] Modules linked in: ...
[ 6427.079950] Pid: 6613, comm: ifconfig Tainted: G           O 3.3.3 #5
[ 6427.079954] Call Trace:
[ 6427.079963]  [<c10337a2>] warn_slowpath_common+0x72/0xa0
[ 6427.079982]  [<c1033873>] warn_slowpath_fmt+0x33/0x40
[ 6427.079988]  [<c12dcb77>] check_unmap+0x727/0x7a0
[ 6427.079995]  [<c12dcdaa>] debug_dma_unmap_page+0x5a/0x80
[ 6427.080024]  [<fe2312ac>] iwlagn_unmap_tfd+0x12c/0x180 [iwlwifi]
[ 6427.080048]  [<fe231349>] iwlagn_txq_free_tfd+0x49/0xb0 [iwlwifi]
[ 6427.080071]  [<fe228e37>] iwl_tx_queue_unmap+0x67/0x90 [iwlwifi]
[ 6427.080095]  [<fe22d221>] iwl_trans_pcie_stop_device+0x341/0x7b0 [iwlwifi]
[ 6427.080113]  [<fe204b0e>] iwl_down+0x17e/0x260 [iwlwifi]
[ 6427.080132]  [<fe20efec>] iwlagn_mac_stop+0x6c/0xf0 [iwlwifi]
[ 6427.080168]  [<fd8480ce>] ieee80211_stop_device+0x5e/0x190 [mac80211]
[ 6427.080198]  [<fd833208>] ieee80211_do_stop+0x288/0x620 [mac80211]
[ 6427.080243]  [<fd8335b7>] ieee80211_stop+0x17/0x20 [mac80211]
[ 6427.080250]  [<c148dac1>] __dev_close_many+0x81/0xd0
[ 6427.080270]  [<c148db3d>] __dev_close+0x2d/0x50
[ 6427.080276]  [<c148d152>] __dev_change_flags+0x82/0x150
[ 6427.080282]  [<c148e3e3>] dev_change_flags+0x23/0x60
[ 6427.080289]  [<c14f6320>] devinet_ioctl+0x6a0/0x770
[ 6427.080296]  [<c14f8705>] inet_ioctl+0x95/0xb0
[ 6427.080304]  [<c147a0f0>] sock_ioctl+0x70/0x270

Cc: stable@vger.kernel.org
Reported-by: Antonio Quartulli <ordex@autistici.org>
Tested-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Reviewed-by: Wey-Yi W Guy <wey-yi.w.guy@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>