net: devlink: introduce a flag to indicate devlink port being registered

Instead of relying on devlink pointer not being initialized, introduce
an extra flag to indicate if devlink port is registered. This is needed
as later on devlink pointer is going to be initialized even in case
devlink port is not registered yet.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>