beep/mixer code: Code police a bit.

/firmware is in the #include path with makes #include "asm/beep.c" sufficient.

Add a comment to generic beep code and make another express intent better.

Change-Id: I587cd704478b894785927bdfe2e647e28614df62

apps/beep.c

@@ -27,7 +27,7 @@
 #include "misc.h"
 
 /** Beep generation, CPU optimized **/
-#include "../firmware/asm/beep.c"
+#include "asm/beep.c"
 
 static uint32_t beep_phase;     /* Phase of square wave generator */
 static uint32_t beep_step;      /* Step of square wave generator on each sample */

firmware/asm/beep.c

@@ -24,6 +24,7 @@
 #include "m68k/beep.c"
 #else /* Generic */
 
+/* Actually output samples into beep_buf */
 static FORCE_INLINE void beep_generate(int16_t *out, int count,
                                        uint32_t *phase, uint32_t step,
                                        int16_t amplitude)

firmware/pcm_mixer.c

@@ -271,7 +271,9 @@ void mixer_channel_play_data(enum pcm_mixer_channel channel,
     {
         /* Initial buffer not passed - call the callback now */
         pcm_play_lock();
-        mixer_deactivate_channel(chan); /* Protect chan struct if active */
+        mixer_deactivate_channel(chan); /* Protect chan struct if active;
+                                           may also be same callback which
+                                           must not be reentered */
         pcm_play_unlock(); /* Allow playback while doing callback */
 
         size = 0;