ALSA: mixart: Proper endian notations

The miXart driver deals with big-endian values as raw data, while it
declares most of variables as u32.  This leads to sparse warnings like
  sound/pci/mixart/mixart.c:1203:23: warning: cast to restricted __be32

Fix them by properly defining the structs and add the explicit cast to
macros.

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