iwlwifi: mvm: fix array out of bounds reference

When starting aggregation, the code checks the status of the queue
allocated to the aggregation tid, which might not yet be allocated
and thus the queue index may be invalid.
Fix this by reserving a new queue in case the queue id is invalid.

While at it, clean up some unreachable code (a condition that is
already handled earlier) and remove all the non-DQA comments since
non-DQA mode is no longer supported.

Fixes: cf961e16 ("iwlwifi: mvm: support dqa-mode agg on non-shared queue")
Signed-off-by: Avraham Stern <avraham.stern@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>