ALSA: usb-audio: Fixing usage of plain int instead of NULL

As reported by kbuild test robot, mixer quirks for RME Babyface
Pro used plain integer instead of NULL.

Fixes: 3e8f3bd0

 ("ALSA: usb-audio: RME Babyface Pro mixer patch")
Signed-off-by: Thomas Ebeling <penguins@bollie.de>
Reported-by: kbuild test robot <lkp@intel.com>
Link: https://lore.kernel.org/r/20200529173248.zzawijfvw73kzjxt@bollie.ca9.eu
Signed-off-by: Takashi Iwai <tiwai@suse.de>