iwlwifi: fix warning from ieee80211_stop_tx_ba_cb_irqsafe argument change

CC [M]  drivers/net/wireless/iwlwifi/iwl-tx.o
drivers/net/wireless/iwlwifi/iwl-tx.c: In function ‘iwl_tx_agg_stop’:
drivers/net/wireless/iwlwifi/iwl-tx.c:1356: warning: passing argument 1 of ‘ieee80211_stop_tx_ba_cb_irqsafe’ from incompatible pointer type
include/net/mac80211.h:2128: note: expected ‘struct ieee80211_vif *’ but argument is of type ‘struct ieee80211_hw *’

Signed-off-by: John W. Linville <linville@tuxdriver.com>