iwlwifi: mvm: clear tx queue id when unreserving aggregation queue

When a queue is reserved for aggregation, the queue id is assigned
to the tid_data. This is fine since iwl_mvm_sta_tx_agg_oper()
takes care of allocating the queue before actual tx starts.
When the reservation is cancelled (e.g. when the AP declined the
aggregation request) the tid_data is not cleared. As a result,
following tx for this tid was trying to use an unallocated queue.

Fix this by setting the txq_id for the tid to invalid when unreserving
the queue.

Signed-off-by: Avraham Stern <avraham.stern@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>