ASoC: Optimize switch/case in magician.c

Use default to optimize the switch/case in magicial_playback_hw_params(),
which also fixes the compile warnings below:
  sound/soc/pxa/magician.c:89: warning: 'acds' may be used uninitialized in this function
  sound/soc/pxa/magician.c:89: warning: 'acps' may be used uninitialized in this function

Signed-off-by: Takashi Iwai <tiwai@suse.de>