iwlwifi: mvm: free reserved queue on STA removal

When a STA is removed in DQA mode, if no traffic went through
its reserved queue, the txq continues to be marked as
reserved and no STA can use it.

Make sure that in such a case the reserved queue is marked
as free when the STA is removed.

Fixes: commit 24afba76 ("iwlwifi: mvm: support bss dynamic alloc/dealloc of queues")
Signed-off-by: Liad Kaufman <liad.kaufman@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>