ALSA: au88x0: Give comment for vortex_wtdma_bufshift() issue

The check of the return value from vortex_wtdma_bufshft() in
vortex_interrupt() is useless as it executes no code.  Actually the
return value is intentionally ignored because the delta calculation
for wavetable doesn't work always correctly.  For avoiding the
confusion, a comment is added and the superfluous if () is removed.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>