iwlwifi: pcie: free the TSO page when a Tx queue is unmapped on A000 devices

When we unmap a non-empty Tx queue, we need to free the
pages that we allocated for the headers in TSO flows.
This code existed for the 9000 device family, but somehow
it got left out when the new Tx path for the A000 devices
was written.

Fixes: 2b0c5946d9ed ("iwlwifi: pcie: introduce a000 TX queues management")
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>