olsrd: fix respawn_threshold lookup in the IPv6 init script

olsrd: fix respawn_threshold lookup in the IPv6 init script

The UCI option is spelled respawn_threshold, as documented in
files/olsrd6.config and as read by files/olsrd4.init, but the IPv6
init script looks up _respawn_threshold. The lookup never matches,
so a user-configured value is silently ignored and the built-in
default of 3600 is always used.

Reported-by: openwrt-ai[bot]
Signed-off-by: Josef Schlehofer <redacted>
Co-authored-by: Claude Fable 5 <redacted>