ALSA: sparc: Mark expected switch fall-throughs

Mark switch cases where we are expecting to fall through.

This patch fixes the following warnings (Building: sparc64):

sound/sparc/dbri.c: In function ‘reverse_bytes’:
sound/sparc/dbri.c:582:5: warning: this statement may fall through [-Wimplicit-fallthrough=]
   b = ((b & 0xffff0000) >> 16) | ((b & 0x0000ffff) << 16);
   ~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
sound/sparc/dbri.c:583:2: note: here
  case 16:
  ^~~~
sound/sparc/dbri.c:584:5: warning: this statement may fall through [-Wimplicit-fallthrough=]
   b = ((b & 0xff00ff00) >> 8) | ((b & 0x00ff00ff) << 8);
   ~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
sound/sparc/dbri.c:585:2: note: here
  case 8:
  ^~~~
sound/sparc/dbri.c:586:5: warning: this statement may fall through [-Wimplicit-fallthrough=]
   b = ((b & 0xf0f0f0f0) >> 4) | ((b & 0x0f0f0f0f) << 4);
   ~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
sound/sparc/dbri.c:587:2: note: here
  case 4:
  ^~~~
sound/sparc/dbri.c:588:5: warning: this statement may fall through [-Wimplicit-fallthrough=]
   b = ((b & 0xcccccccc) >> 2) | ((b & 0x33333333) << 2);
   ~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
sound/sparc/dbri.c:589:2: note: here
  case 2:
  ^~~~

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>