ALSA: compress: fix the struct alignment to 4 bytes

In 64bit systems the compiler can default align to 8bytes causing mis-match with
32bit usermode. Avoid this is future by ensuring all the structures shared with
usermode are packed and aligned to 4 bytes irrespective of arch used

[coding style fixes by tiwai]

Signed-off-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>