iwlwifi: make iwl4965_mac_conf_tx in atomic context

This patch fixes iwl4965_mac_conf_tx. A mutex was taken in atomic context
leading to Oops. This patch removes the mutex and extends the hold
priv->lock. None of the field of QOS is accessed without priv->lock held.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>