ALSA: usb-audio: Fix potential out-of-bounds shift

syzbot spotted a potential out-of-bounds shift in the USB-audio format
parser that receives the arbitrary shift value from the USB
descriptor.

Add a range check for avoiding the undefined behavior.

Reported-by:  <syzbot+df7dc146ebdd6435eea3@syzkaller.appspotmail.com>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20201209084552.17109-1-tiwai@suse.de


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