iwlwifi: grab NIC access when disabling aggregations

This patch grabs NIC access inside iwl4965_tx_queue_agg_disable, instead of the
caller doing it.
The caller must still hold priv->lock when calling the function.

Signed-off-by: Max Stepanov <max.stepanov@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Ron Rindjunsky <ron.rindjunsky@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>