ALSA: riptide: Fix PCM format type conversion

The PCM format type is with __bitwise, hence it needs to be explicitly
declared as snd_pcm_format_t, as warned by sparse:
  sound/pci/riptide/riptide.c:1028:34: warning: incorrect type in argument 1 (different base types)
  sound/pci/riptide/riptide.c:1028:34:    expected restricted snd_pcm_format_t [usertype] format
  sound/pci/riptide/riptide.c:1028:34:    got unsigned char [unsigned] format

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