res_rtp_asterisk: Fix placement of txcount increment

Commit 1bce690c was incrementing txcount
for rtcp packets as well as rtp packets and that was causing sender reports
to be generated instead of receiver reports in cases where no rtp was actually
being sent.

Moved the txcount increment from __rtp_sento, which handles both rtp and rtcp,
to rtp_sento which only handles rtp packets.

Discovered by the hep/rtcp-receiver test.

Change-Id: Ie442e4bb947a68847a676497021ba10ffaf376d5