wifi: iwlwifi: mvm: don't wait for tx queues if firmware is dead

In the Linux kernel, the following vulnerability has been resolved: wifi: iwlwifi: mvm: don't wait for tx queues if firmware is dead There is a WARNING in iwl_trans_wait_tx_queues_empty() (that was recently converted from just a message), that can be hit if we wait for TX queues to become empty after firmware died. Clearly, we can't expect anything from the firmware after it's declared dead. Don't call iwl_trans_wait_tx_queues_empty() in this case. While it could be a good idea to stop the flow earlier, the flush functions do some maintenance work that is not related to the firmware, so keep that part of the code running even when the firmware is not running. [edit commit message]