res/res_rtp_asterisk: smoother can cause wrong timestamps if dtmf happen

Delivery timeval in the smoother object will fall behind while a DTMF is
being generated.  This can eventually lead to invalid rtp timestamps.
To prevent this from happening the smoother needs to be reset after every
DTMF to keep the timing up to date.

ASTERISK-28303 #close

Change-Id: Iaba3f7b428ebd72a4caa90e13b829ab4f088310f