batman-adv-port: Increase MTU to 1560 as suggested by the batman guys

"The MTU of interface batp0 is too small (1528) to handle the transport
of batman-adv packets. Packets going over this interface will be
fragmented on layer2 which could impact the performance.
Setting the MTU to 1560 would solve the problem."
batman-adv-port: Fix wrong function call