mac80211: fix start_seq_num in Rx reorder offload

sta->last_seq_ctrl is the seq_ctrl field from the last header
seen, need to shift it 4 bits to extract the sequence number.
Otherwise the ieee80211_sn_less() check at the top of
ieee80211_sta_manage_reorder_buf drops frames until the sequence
number catches up.

Cc: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Denton Gentry <denton.gentry@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>