ALSA: soc - Fix compile warnings in wm8903.c

Hide annoying uninitialized warnings:
  sound/soc/codecs/wm8903.c:382: warning: ‘reg’ may be used uninitialized in this function
  sound/soc/codecs/wm8903.c:383: warning: ‘shift’ may be used uninitialized in this function

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