ASoC: pcm: Fix unused variable warning

sound/soc/soc-pcm.c: In function ‘soc_pcm_set_msb’:
sound/soc/soc-pcm.c:307:11: warning: unused variable ‘i’ [-Wunused-variable]

Fixes: 0e2a3751 ('ASoC: pcm: Use wildcard msbits constraints')
Signed-off-by: Takashi Iwai <tiwai@suse.de>