chan_pjsip: DTMF Mode auto_info fallback lead to both inband and info

When the dtmf_mode on an endpoint is configured as "auto_info"
Asterisk will produce an inband DTMF tone alongside an INFO
message when sending DTMF.

ASTERISK-28371

Change-Id: I1380b82f006e110a1b83fbb50c9873edd13a5d9a