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

channels/chan_pjsip.c

@@ -1758,7 +1758,7 @@ static int chan_pjsip_digit_begin(struct ast_channel *chan, char digit)
 		break;
 	case AST_SIP_DTMF_AUTO_INFO:
 		if (!media || !media->rtp || (ast_rtp_instance_dtmf_mode_get(media->rtp) == AST_RTP_DTMF_MODE_NONE)) {
-			return -1;
+			return 0;
 		}
 		ast_rtp_instance_dtmf_begin(media->rtp, digit);
 		break;