gluon-client-bridge: set ra_holdoff interval to 30 seconds (#1597)

Allow odhcp6c to fork the script to handle router
advertisments in 30 seconds intervals. This is the value
that was previously used in Gluon v2018.1 / LEDE 17.01.

The default value is 3 seconds and while it is RFC compliant
it can put alot of pressure on even moderately sized devices.

Signed-off-by: Martin Weinelt <martin@darmstadt.freifunk.net>