mt76: fix fix ampdu locking

The current ampdu locking code does not unlock its mutex in the early
return case. This patch fixes it.

Signed-off-by: Markus Theil <markus.theil@tu-ilmenau.de>
Acked-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>