ALSA: pcm: Fix sparse warning wrt PCM format type

The PCM format type is with __bitwise, hence it needs the explicit
cast with __force.  It's ugly, but there is a reason for that cost...

This fixes the sparse warning:
  sound/core/oss/pcm_oss.c:1854:55: warning: incorrect type in argument 1 (different base types)

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