ALSA: xen: Use standard pcm_format_to_bits() for ALSA format bits

The open codes with the bit shift in xen_snd_front_alsa.c give sparse
warnings as the PCM format type is with __bitwise.
There is already a standard macro to get the format bits, so let's use
it instead.

This fixes sparse warnings like:
  sound/xen/xen_snd_front_alsa.c:191:47: warning: restricted snd_pcm_format_t degrades to integer

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