ALSA: pcm: Remove yet superfluous WARN_ON()

muldiv32() contains a snd_BUG_ON() (which is morphed as WARN_ON() with
debug option) for checking the case of 0 / 0.  This would be helpful
if this happens only as a logical error; however, since the hw refine
is performed with any data set provided by user, the inconsistent
values that can trigger such a condition might be passed easily.
Actually, syzbot caught this by passing some zero'ed old hw_params
ioctl.

So, having snd_BUG_ON() there is simply superfluous and rather
harmful to give unnecessary confusions.  Let's get rid of it.

Reported-by:  <syzbot+7e6ee55011deeebce15d@syzkaller.appspotmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>