Ponsim: UNI port is hardcoded to 128

When the Ponsim OLT traps a packet to the OF controller, the Ponsim adapter in VOLTHA needs to specify the packet's IN_PORT on the logical device.  If the subscriber flows have not yet been set up, then the VLAN_VID of the packet is the IN_PORT; otherwise the VLAN_VID is the subscriber C-tag and the adapter needs to look up the UNI port for that C-tag.

Currently the Ponsim adapter hardcodes the lookup to 128, the default UNI port:

https://github.com/opencord/voltha/blob/f365ea3cad562cf4218a97fd721ae063beb778b4/voltha/adapters/ponsim_olt/ponsim_olt.py#L713-L721

Fix this.  In the process simplify the Ponsim adapter code by deriving the C-tag to UNI port mapping from the ONU flows.