freetdm: add channel availability rate for BRI (may be used by other protocols if needed)

libs/freetdm/src/include/private/ftdm_core.h

@@ -407,6 +407,7 @@ struct ftdm_channel {
 	uint8_t txgain_table[FTDM_GAINS_TABLE_SIZE];
 	float rxgain;
 	float txgain;
+	int availability_rate;
 	void *user_private;
 };
 

libs/freetdm/src/include/private/ftdm_types.h

@@ -175,6 +175,7 @@ typedef enum {
 	FTDM_SPAN_STOP_THREAD = (1 << 5),
 	FTDM_SPAN_USE_CHAN_QUEUE = (1 << 6),
 	FTDM_SPAN_SUGGEST_CHAN_ID = (1 << 7),
+	FTDM_SPAN_USE_AV_RATE = (1 << 8),
 } ftdm_span_flag_t;
 
 /*! \brief Channel supported features */