net: hsr: require valid EOT supervision TLV

net: hsr: require valid EOT supervision TLV

Supervision frames are only valid if terminated with a zero-length EOT
TLV. The current check fails to reject non-EOT entries as the terminal
TLV, potentially allowing malformed supervision traffic.

Fix this by strictly requiring the terminal TLV to be HSR_TLV_EOT with
a length of zero.

Signed-off-by: Luka Gejak <luka.gejak@linux.dev>
Reviewed-by: Fernando Fernandez Mancera <fmancera@suse.de>
Link: https://patch.msgid.link/20260523130420.62144-1-luka.gejak@linux.dev
Signed-off-by: Jakub Kicinski <kuba@kernel.org>