[STF] only capture packets from the main connection

Otherwise it is more than likely to disturb the packet order.

Change-Id: Ia391878b87678adb0923d8ea510257cbf57ea33a
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1952
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>