iwlwifi: mvm: avoid possible access out of array.

The value in txq_id can be out of array scope,
validate it before accessing the array.

Signed-off-by: Mordechay Goodstein <mordechay.goodstein@intel.com>
Fixes: cf961e16 ("iwlwifi: mvm: support dqa-mode agg on non-shared queue")
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>