Revise the PCM callback system after adding multichannel audio.

Additional status callback is added to pcm_play/rec_data instead of
using a special function to set it. Status includes DMA error
reporting to the status callback. Playback and recording callback
become more alike except playback uses "const void **addr" (because
the data should not be altered) and recording  uses "void **addr".
"const" is put in place throughout where appropriate.

Most changes are fairly trivial. One that should be checked in
particular because it isn't so much is telechips, if anyone cares to
bother. PP5002 is not so trivial either but that tested as working.

Change-Id: I4928d69b3b3be7fb93e259f81635232df9bd1df2
Reviewed-on: http://gerrit.rockbox.org/166
Reviewed-by: Michael Sevakis <jethead71@rockbox.org>
Tested-by: Michael Sevakis <jethead71@rockbox.org>

apps/beep.c

@@ -46,7 +46,7 @@ static int16_t beep_buf[BEEP_BUF_COUNT*2] IBSS_ATTR __attribute__((aligned(4)));
 /* Callback to generate the beep frames - also don't want inlining of
    call below in beep_play */
 static void __attribute__((noinline))
-beep_get_more(unsigned char **start, size_t *size)
+beep_get_more(const void **start, size_t *size)
 {
     int count = beep_count;
 
@@ -87,7 +87,7 @@ void beep_play(unsigned int frequency, unsigned int duration,
 #endif
 
     /* If it fits - avoid cb overhead */
-    unsigned char *start;
+    const void *start;
     size_t size;
 
     /* Generate first frame here */