realtek: dsa: Clarify meaning of secondary lag member variable

realtek: dsa: Clarify meaning of secondary lag member variable

The name "is_lagmember" implies that the port is part of a LAG. But this
information is already stored in lagmembers. In reality, it is stored the
non-primary LAG members. Renaming it accordingly, makes the code a lot more
readable

Also the type (u32 array) looks like it would contain some kind of large id
(like the group ID). But it only stores a single bit. It is more appropriate
to just use a single bit per port.

Signed-off-by: Sven Eckelmann <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/20707
Signed-off-by: Hauke Mehrtens <[email protected]>