iwlwifi: mvm: fix NVM parsing error path

If NVM parsing fails and returns NULL, we continue in
the code flow and eventually crash accessing the NULL
pointer. Return an error from iwl_nvm_init() if the
parsing failed.

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