Revert "ALSA: usb-audio: Fix potential zero-division at parsing FU"

The commit 8428a8eb ("ALSA: usb-audio: Fix potential zero-division
at parsing FU") is utterly bogus and breaks the case with csize=1
instead of fixing anything.  Just take it back again.

Reported-by: Jörg Otte <jrg.otte@gmail.com>
Fixes: 8428a8eb ("ALSA: usb-audio: Fix potential zero-division at parsing FU"
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>