wcn36xx: let device generate qos seq numbers

wcn36xx currently sends an incorrect sequence number into the BA session
setup firmware command: it should be saving or updating the ssn in the
TX_START ampdu_action callback instead of waiting until TX_OPERATIONAL.
However, we can sidestep the issue by letting the hardware generate the
sequence numbers for QoS frames, as is done in prima, so do that.

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>