net: devlink: introduce port registered assert helper and use it

Instead of checking devlink_port->devlink pointer for not being NULL
which indicates that devlink port is registered, put this check to new
pair of helpers similar to what we have for devlink and use them in
other functions.

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