The previous code naively looked for a `t` in the entire port spec,
wrongly matching untagged ports having a `t` in their name, such
as `eth0`.
Rework the logic to be more strict when parsing the port member
specification to avoid this issue.
Signed-off-by: Jo-Philipp Wich <jo@mein.io>
(cherry picked from commit 01eac366f69ba58c090ddc2cb70aeb069adbdd5a)
4c05263573