gluon-mesh-batman-adv: increase bridge multicast querier robustness to 9

The RFC standard multicast querier interval is 120s. Our querier uses in
interval of 20s for better support of roaming clients, but our robustness
setting of 3 leads to external queriers using the standard interval to be
timeout after only 60s, leading to frequent "querier appeared/disappeared"
messages. Increase robustness so that external queriers with any interval
<180s are supported.