mt76: drop rcu read lock in mt76_rx_aggr_stop

A rcu read locked section is not allowed to sleep, and the rcu lock here
isn't actually necessary, because we're holding dev->mutex.
Fixes an issue when the tid work item is still running while freeing
a station or stopping the aggregation session

Signed-off-by: Felix Fietkau <nbd@nbd.name>