iwlwifi: warn if firmware image doesn't exist

If the firmware image that we attempt to load doesn't
actually exist we have a broken firmware file or other
code not checking things correctly, so warn in such a
case. Also avoid assigning cur_ucode/ucode_loaded then.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>