devlink: append split port number to the port name

Instead of doing sprintf twice in case the port is split or not, append
the split port suffix in case the port is split.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20210527104819.789840-1-jiri@resnulli.us
Signed-off-by: Jakub Kicinski <kuba@kernel.org>