ALSA: usb-audio: silence a static checker warning

We recently made "format" a u64 variable so now static checkers complain
that this shift will wrap around if format is more than 31.  I don't
think it makes a difference for runtime, but it's simple to silence the
warning.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>