iwlwifi: fix possible read attempt on ucode that is not available

This fixes a NULL pointer dereference that can occur when the
ucode is not loaded at the time __iwl_up is called.

The problem was reported at http://kerneloops.org/raw.php?rawid=2765&msgid=

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>