devlink: Don't throw an error if flash notification sent before devlink visible

The mlxsw driver calls to various devlink flash routines even before
users can get any access to the devlink instance itself. For example,
mlxsw_core_fw_rev_validate() one of such functions.

__mlxsw_core_bus_device_register
 -> mlxsw_core_fw_rev_validate
  -> mlxsw_core_fw_flash
   -> mlxfw_firmware_flash
    -> mlxfw_status_notify
     -> devlink_flash_update_status_notify
      -> __devlink_flash_update_notify
       -> WARN_ON(...)

It causes to the WARN_ON to trigger warning about devlink not registered.

Fixes: cf530217 ("devlink: Notify users when objects are accessible")
Reported-by: Danielle Ratson <danieller@nvidia.com>
Tested-by: Danielle Ratson <danieller@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>