mt76: mt7603: fix sequence number assignment

If the MT_TXD3_SN_VALID flag is not set in the tx descriptor, the hardware
assigns the sequence number. However, the rest of the code assumes that the
sequence number specified in the 802.11 header gets transmitted.
This was causing issues with the aggregation setup, which worked for the
initial one (where the sequence numbers were still close), but not for
further teardown/re-establishing of sessions.

Additionally, the overwrite of the TID sequence number in WTBL2 was resetting
the hardware assigned sequence numbers, causing them to drift further apart.

Fix this by using the software assigned sequence numbers

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>