ALSA: bt87x: Proper endian notations

The RISC data in bt87x is in little-endian, hence we should define it
with __le32 properly.

Spotted by sparse, a warning like:
  sound/pci/bt87x.c:240:17: warning: incorrect type in assignment (different base types)

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