iwlwifi: check IEEE80211_TX_STAT_AMPDU for agg pkt

when perform rate scaling, in tx status function, checking for
IEEE80211_TX_STAT_AMPDU flag instead of IEEE_TX_CTL_AMPDU flag to perform
AMPDU rate scaling operation.

IEEE80211_TX_CTL_AMPDU was set by mac80211 for aggregation pkt. But when
iwlwifi receive the tx status reply, it reset the flag to following
	info->flags = IEEE80211_TX_STAT_ACK;
	info->flags |= IEEE80211_TX_STAT_AMPDU;
it causes the rate-scaling to not work for aggregation pkt if we checking
for IEEE80211_TX_CTL_AMPDU flag.

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>