res_pjsip_sdp_rtp: wrong bridge chosen when the DTMF mode is not compatible

res_pjsip_sdp_rtp: wrong bridge chosen when the DTMF mode is not compatible

A native rtp bridge was being chosen (it shouldn't have been) when using two
pjsip channels with incompatible DTMF modes.  This patch sets the rtp instance
property, AST_RTP_PROPERTY_DTMF, for the appropriate DTMF mode(s) for pjsip.
It was not being set before, meaning all DTMF modes for pjsip were being treated
as compatible, thus native bridging would be chosen as the bridge type when it
shouldn't have been.

ASTERISK-24459 #close
Reported by: Yaniv Simhi
Review: https://reviewboard.asterisk.org/r/4265/

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/13@429763 65c4cc65-6c06-0410-ace0-fbb531ad65f3