ALSA: au88xx: Add fall-through annotations

As a preparatory patch for the upcoming -Wimplicit-fallthrough
compiler checks, add the "fall through" annotation in au88xx driver.

Although the usages in both both functions vortex_adbdma_setbuffers()
and vortex_wtdma_setbuffers() look a bit suspicious, we keep the
behavior as before, just to be safer.  If it were broken, we should
have already received bug reports.

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