ftmod_misdn: Fix brown paperbag logic bug in handle_b_channel_event()

Yeah, right, discard the incoming audio data when the b-channel is active...

Reverse the check, so ftdm_read() on an active mISDN b-channel will
be able to read audio data from the RX socketpair and not kill
the call from repeatedly timing out in poll().

Fixes:
    [WARNING] mod_freetdm.c:775 Too many timeouts while waiting I/O in channel FreeTDM/1:1/XX device 1:1!
    [ERR] mod_freetdm.c:820 clearing IO in channel FreeTDM/1:1/XX device 1:1!
Signed-off-by: Stefan Knoblich <stkn@openisdn.net>

libs/freetdm/src/ftmod/ftmod_misdn/ftmod_misdn.c

@@ -2298,7 +2298,7 @@ static ftdm_status_t handle_b_channel_event(ftdm_channel_t *chan)
 		char *data  = buf    + MISDN_HEADER_LEN;
 
 		/* Discard incoming audio if not active */
-		if (!priv->active) {
+		if (priv->active) {
 			/* Convert audio data */
 			misdn_convert_audio_bits(data, datalen);