ALSA: riptide: Properly endian notations

The SG descriptor of Riptide contains the little-endian values, hence
we need to define with __le32 properly.  This fixes sparse warnings
like:
  sound/pci/riptide/riptide.c:1112:40: warning: cast to restricted __le32

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