iwlwifi: Don't unlock priv->mutex if it isn't locked

Commit b716bb91 ("iwlwifi: Cancel scanning upon association") moved the
test of priv->vif in iwl{3945,4964}_mac_config_interface() outside of
where priv->mutex is held, but still tries to do mutex_unlock() on
return.  This is clearly wrong and triggers a nasty lockdep warning when
this codepath is triggered.  Fix this by removing the mutex_unlock().

Signed-off-by: Roland Dreier <rolandd@cisco.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>