iwlwifi: mvm: fix flushing not started aggregation sessions

When a not fully started aggregation session is destroyed
and flushed, we get a warning, e.g.

  WARNING: at drivers/net/wireless/iwlwifi/pcie/tx.c:1142 iwl_trans_pcie_txq_disable+0x11c/0x160
  queue 16 not used
  Modules linked in: [...]
  Pid: 5135, comm: hostapd Tainted: G        W  O 3.5.0 #10
  Call Trace:
  wlan0: driver sets block=0 for sta 00:03:7f:10:44:d3
   [<ffffffff81036492>] warn_slowpath_common+0x72/0xa0
   [<ffffffff81036577>] warn_slowpath_fmt+0x47/0x50
   [<ffffffffa0368d6c>] iwl_trans_pcie_txq_disable+0x11c/0x160 [iwlwifi]
   [<ffffffffa03a2099>] iwl_mvm_sta_tx_agg_flush+0xe9/0x150 [iwlmvm]
   [<ffffffffa0396c43>] iwl_mvm_mac_ampdu_action+0xf3/0x1e0 [iwlmvm]
   [<ffffffffa0293ad3>] ___ieee80211_stop_tx_ba_session+0x193/0x920 [mac80211]
   [<ffffffffa0294ed8>] __ieee80211_stop_tx_ba_session+0x48/0x70 [mac80211]
   [<ffffffffa029159f>] ieee80211_sta_tear_down_BA_sessions+0x4f/0x80 [mac80211]
   [<ffffffffa028a686>] __sta_info_destroy+0x66/0x370 [mac80211]
   [<ffffffffa028abb4>] sta_info_destroy_addr_bss+0x44/0x70 [mac80211]
   [<ffffffffa02a3e26>] ieee80211_del_station+0x26/0x50 [mac80211]
   [<ffffffffa01e6395>] nl80211_del_station+0x85/0x200 [cfg80211]

when a station deauthenticated from us without fully setting
up the aggregation session.

Fix this by checking the aggregation state before removing
the hardware queue.

Cc: stable@vger.kernel.org
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>