mt76: do not send BAR frame on tx aggregation flush stop

There is no need to send a BAR frame after stopping aggregation, and doing
so could lead to sending class 3 frames after deauthentication from an AP

Signed-off-by: Balakrishna Bandi <b.balakrishna@globaledgesoft.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>