nfp: devlink: set unknown fw_load_policy

If the 'app_fw_from_flash' HWinfo key is invalid, set the
'fw_load_policy' devlink parameter value to unknown.

Suggested-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Dirk van der Merwe <dirk.vandermerwe@netronome.com>
Signed-off-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>