G.729B: Explicitly use 80:10 ratio in expressions, instead of 8:1

Once G.729B support is added, these expressions are about to get a bit
more complicated, to account for the different size of SID frames.  By
switching to a 80:10 ratio right away, we can get a head start and
reduce the complexity of the last patch.  (Not to mention that this
makes things a bit more explicit as a bonus.)

For the record, there is a reason behind performing the division before
the multiplication, which will become apparent in the final patch.  Note
that the quotient is guaranteed to be integral, due to the assertions.

src/audio/audio_decoder.cpp

@@ -572,14 +572,14 @@ t_g729a_audio_decoder::~t_g729a_audio_decoder()
 
 uint16 t_g729a_audio_decoder::get_ptime(uint16 payload_size) const
 {
-	return (payload_size * 8) / (audio_sample_rate(_codec) / 1000);
+	return ((payload_size / 10) * 80) / (audio_sample_rate(_codec) / 1000);
 }
 
 uint16 t_g729a_audio_decoder::decode(uint8 *payload, uint16 payload_size,
 		int16 *pcm_buf, uint16 pcm_buf_size)
 {
 	assert((payload_size % 10) == 0);
-	assert(pcm_buf_size >= payload_size*8);
+	assert(pcm_buf_size >= ((payload_size / 10) * 80));
 
 	uint8 frame_size = 10;
 	uint16 result_size = 0;

src/audio/audio_encoder.cpp

@@ -491,7 +491,7 @@ uint16 t_g729a_audio_encoder::encode(int16 *sample_buf, uint16 nsamples,
 		uint8 *payload, uint16 payload_size, bool &silence)
 {
 	assert ((nsamples % 80) == 0);
-	assert (payload_size >= (nsamples/8));
+	assert (payload_size >= ((nsamples / 80) * 10));
 
 	silence = false;