mlx5: shd: Gracefully avoid shared devlink creation when no usable SN is found

mlx5: shd: Gracefully avoid shared devlink creation when no usable SN is found

On some HW, not even fall-back "SERIALNO" is found in VPD data. Unify
the behavior with the case there are no VPD data at all and avoid
creation of shared devlink instance.

Fixes: 2a8c8a03f306 ("net/mlx5: Add a shared devlink instance for PFs on same chip")
Reported-by: Adam Young <admiyo@amperemail.onmicrosoft.com>
Closes: https://lore.kernel.org/all/bab5b6bc-aa42-4af1-80d1-e56bcef06bc2@amperemail.onmicrosoft.com/
Reported-by: Ben Copeland <ben.copeland@linaro.org>
Closes: https://lore.kernel.org/all/20260324151014.860376-1-ben.copeland@linaro.org/
Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Tested-by: Ben Copeland <ben.copeland@linaro.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20260325152801.236343-1-jiri@resnulli.us
Signed-off-by: Jakub Kicinski <kuba@kernel.org>