ALSA: usb-audio: Move playback_first flag into quirk_flags

The snd_usb_audio.playback_first flag is used by the implicit feedback
mode handling, and this can be also moved to quirk_flags.

Link: https://lore.kernel.org/r/20210729073855.19043-6-tiwai@suse.de


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