Fix match metadata in FTTB Subscriber downstream flows

Currently the match metadata in the FTTB Subscriber downstream flows is getting set to the C-tag, this should rather be set to the S-tag of the subscriber. The S-tag is what is unique within an OLT for the FTTB subscribers and so this should be used, else we will have duplicate flow rules for different FTTB subscribers as the might use the same C-tag value.