devlink: add unknown 'fw_load_policy' value

Similar to the 'reset_dev_on_drv_probe' devlink parameter, it is useful
to have an unknown value which can be used by drivers to report that the
hardware value isn't recognized or is otherwise invalid instead of
failing the operation.

This is especially useful for u8/enum parameters.

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>