ALSA: atmel: Remove invalid "fall through" comments

The comments about fall through in sound/atmel/ac97.c are just
superfluous and rather confusing.  Let's remove them.

Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20200709111750.8337-2-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>