txtid->seq_start may not always be up to date, when there is HT non-AMPDU
traffic just before starting an AMPDU session. Relying on txtid->seq_next
is better, since it is also used to generate the sequence numbers for
all QoS data frames.
Signed-off-by: Felix Fietkau <nbd@openwrt.org> Signed-off-by: John W. Linville <linville@tuxdriver.com>