iwlwifi: mvm: Configure agg. queue before assigning it to STA

In order to imeplement the extended VI session feature for Miracast, the FW
requires to detect the VI queue. The detection of the VI queue is done when
it is assigned to a STA with ADD_STA command, so by this time the FW expects
the queue to be already configured (mapped to VI AC and aggregation enabled).
Previously, the queue configuration was done after STA modificaton which
broke the extended VI session feature and resulted in higher latencies.
Fix this by calling iwl_mvm_enable_agg_txq before station modification.

Signed-off-by: Andrei Otcheretianski <andrei.otcheretianski@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>