mac80211: fix tx sequence number assignment with software queue + fast-xmit

When using software queueing, tx sequence number assignment happens at
ieee80211_tx_dequeue time, so the fast-xmit codepath must not do that.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>