pcm: support S24_LE format

S24_LE is 24 bit audio in a 32 bit sample size. Also
make the pcm_format_to_bits() function part of the API
since it will be required for tinycap.

include/tinyalsa/asoundlib.h

@@ -150,6 +150,13 @@ int pcm_set_config(struct pcm *pcm, struct pcm_config *config);
 /* Returns a human readable reason for the last error */
 const char *pcm_get_error(struct pcm *pcm);
 
+/* Returns the sample size in bits for a PCM format.
+ * As with ALSA formats, this is the storage size for the format, whereas the
+ * format represents the number of significant bits. For example,
+ * PCM_FORMAT_S24_LE uses 32 bits of storage.
+ */
+unsigned int pcm_format_to_bits(enum pcm_format format);
+
 /* Returns the buffer size (int frames) that should be used for pcm_write. */
 unsigned int pcm_get_buffer_size(struct pcm *pcm);
 unsigned int pcm_frames_to_bytes(struct pcm *pcm, unsigned int frames);

pcm.c

@@ -212,10 +212,11 @@ static unsigned int pcm_format_to_alsa(enum pcm_format format)
     };
 }
 
-static unsigned int pcm_format_to_bits(enum pcm_format format)
+unsigned int pcm_format_to_bits(enum pcm_format format)
 {
     switch (format) {
     case PCM_FORMAT_S32_LE:
+    case PCM_FORMAT_S24_LE:
         return 32;
     default:
     case PCM_FORMAT_S16_LE: