luci-mod-network: fix tagging/pvid state parsing in bridge-vlan ports

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>