ALSA: snd-usb: Fix uninitialized variable usage

Purely cosmetic, but fixes the following build warning.

  CC [M]  sound/usb/quirks.o
sound/usb/quirks.c: In function ‘snd_usb_apply_boot_quirk’:
sound/usb/quirks.c:429:6: warning: ‘err’ may be used uninitialized in this function [-Wuninitialized]

Signed-off-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>