iwlwifi: 8000: fallback to default NVM file

Set a default NVM in case the userspace specifies a file
that doesn't match the hardware version. This allows not
to change the boot scripts when someone replaces the device
with a newer hardware step.

Signed-off-by: Eran Harary <eran.harary@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>