iwlwifi: checking unknown HW type

When deciding NVM type, if the HW type is unknown, report error and exit
with -ENOENT. This check should prevent incorrect behavior by assuming
the wrong NVM type.

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>