ALSA: xen: Remove superfluous fall through comments

The "fall through" comments found in switch-cases in ALSA xen driver
are all superfluous.  The kernel coding style allows the multiple
cases in a row.  Let's remove them.

Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20200709111750.8337-3-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>