iwlwifi: mvm: limit maximum queue appropriately

Due to some hardware issues, queue 31 isn't usable on devices that have
32 queues (7000, 8000, 9000 families), which is correctly reflected in
the configuration and TX queue initialization.

However, the firmware API and queue allocation code assumes that there
are 32 queues, and if something actually attempts to use #31 this leads
to a NULL-pointer dereference since it's not allocated.

Fix this by limiting to 31 in the IWL_MVM_DQA_MAX_DATA_QUEUE, and also
add some code to catch this earlier in the future, if the configuration
changes perhaps.

Cc: stable@vger.kernel.org # v4.9+
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/iwlwifi.20200417100405.98a79be2db6a.I3a4af6b03b87a6bc18db9b1ff9a812f397bee1fc@changeid