iwlwifi: mvm: fix theoretical uninitialized function return value

If we try to write NVM that do not exist, the function will return
uninitialized value. fixed.

Signed-off-by: Eytan Lifshitz <eytan.lifshitz@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>