Rework on audio buffer management

Use function to get samples to be sent to fill audio buffers to a
level. This replaces the function that only shows how much data
is in the buffer.

This way the function itself can control how much data will be
sent.

src/common/uhd.h

@@ -4,5 +4,5 @@ int uhd_start(void);
 void uhd_close(void);
 int uhd_send(float *buff, int num);
 int uhd_receive(float *buff, int max);
-int uhd_get_inbuffer(void);
+int uhd_get_tosend(int latspl);