[ALSA] Fix Oops with PCM OSS sync

The PCM OSS emulation can cause Oops at sync operation due to the wrong
data size calculation.  Typically happening on Sparc64:
	http://lkml.org/lkml/2008/1/24/426



Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>