V4L/DVB (6970): Request snd-usb-audio for devices with Audio Class support

Before this patch, only Vendor Class audio support were loaded. This means that
older em28xx devices won't have digital audio support loaded.

This patch changes the logic to auto load eighter snd-usb-audio, for devices
with USB Audio Class or em28xx-alsa, for devices with USB Vendor Class.

Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>