devlink: Make devlink_register to be void

devlink_register() can't fail and always returns success, but all drivers
are obligated to check returned status anyway. This adds a lot of boilerplate
code to handle impossible flow.

Make devlink_register() void and simplify the drivers that use that
API call.

Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Acked-by: Simon Horman <simon.horman@corigine.com>
Acked-by: Vladimir Oltean <olteanv@gmail.com> # dsa
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>