mac80211: Fix ieee80211_rx_reorder_ampdu: ignore QoS null packets

This patch fixes the check at the entrance to ieee80211_rx_reorder_ampdu.
This check has been broken by 'mac80211: rx.c use new helpers'.

Letting QoS NULL packet in ieee80211_rx_reorder_ampdu led to packet loss in
RX.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>