ALSA: sb: Fix PCM format bit calculation

The PCM format type in snd_pcm_format_t can't be treated as integer
implicitly since it's with __bitwise.  We have already a helper
function to get the bit index of the given type, and use it in each
place instead.

This fixes sparse warnings like:
  sound/isa/sb/sb16_main.c:61:44: warning: restricted snd_pcm_format_t degrades to integer

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