realtek: redo the checks for different protocols.

realtek: redo the checks for different protocols.

Do it iwth an if chain, which is somewhat the equivalent of what
Wireshark does (it uses heuristic dissectors; our ifs do the same checks
that Wireshark's heuristic dissectors do).

That makes it a bit more obvious that the necessary tests are somewhat
weird, thanks to Realtek not just deciding to make the first octet a
protocol discriminator *even for switch tags*.

We also treat a first octet of 0x23 as meanin RLDP, just as Wireshark
does, as some switch chips use 0x23 for loop detection packets.

Shuffle some comments while we're at it (and epand the one for RLDP to
match all the stuff going on with various RLDP packets).