iwlwifi: fix oops in iwl3945_led_brightness_set

fix race between:
ieee80211_open->ieee80211_led_radio->led_trigger_event->led_set_brightness->iwl3945_led_brightness_set
(which assumes that "led->priv" is not NULL)
and
iwl3945_pci_probe->iwl3945_setup_deferred_work->(...)->iwl3945_bg_alive_start->iwl3945_alive_start->iwl3945_led_register->iwl3945_led_register_led
which sets priv field in struct iwl3945_led
after
led->led_dev.brightness_set = iwl3945_led_brightness_set;
(...)
led_classdev_register(device, &led->led_dev);

http://kerneloops.org/guilty.php?guilty=iwl3945_led_brightness_set&version=2.6.25-release&start=1671168&end=1703935&class=oops

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Cc: Zhu Yi <yi.zhu@intel.com>
Cc: Reinette Chatre <reinette.chatre@intel.com>
Cc: Tomas Winkler <tomas.winkler@intel.com>
Cc: linux-wireless@vger.kernel.org
Cc: ipw3945-devel@lists.sourceforge.net
Signed-off-by: John W. Linville <linville@tuxdriver.com>