voip_calls: Fix tap callbacks.

We don't need to call the VoIP tap reset and draw callbacks repeately.
Do so only once from the RTP tap. Packet callbacks should return a
gboolean.

Clean up some function names and make some static.

Change-Id: I5c934ce8ce7f279861e8cc73235bbfc27d7fe622
Reviewed-on: https://code.wireshark.org/review/5396
Reviewed-by: Gerald Combs <gerald@wireshark.org>