Fix for rtp_timestamp having always proper value
There was an audio issue for internal call caused by the inconsistency in frame->ts value. For the internal call the frame->ts is the real timestamp but for external call it was ptime value. The other fix in res_rtp_asterisk.c corrected it so that frame->ts has always the real timestamp value (not ptime), so there is no need to calculate the timestamp in chan_voicemngr.