iwlwifi: mvm: fix L2P BA ressources leak

We didn't release the Rx AMPDU ressources properly.
This bug led to firmware assert after 16 BA sessions.

Cc: <stable@vger.kernel.org> [3.9+]
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>