iwlwifi: mvm: fix uninitialized var while waiting for queues to empty

While waiting for queues to empty,
If txq_id == IWL_MVM_INVALID_QUEUE for all txq_ids,
ret is used uninitialized.
Found by Klocwork.

Fixes: d6d517b7 ("iwlwifi: add wait for tx queue empty")
Signed-off-by: Sharon Dvir <sharon.dvir@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>