ALSA: usb-audio: Pass snd_usb_audio object to quirk functions

A preliminary patch for the later big change.  Just a minor code
refactoring.

Tested-by: Keith Milner <kamilner@superlative.org>
Tested-by: Dylan Robinson <dylan_robinson@motu.com>
Link: https://lore.kernel.org/r/20201123085347.19667-19-tiwai@suse.de


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