ALSA: usb-audio: Move tx_length quirk handling to quirk_flags

There is another quirk for the transfer, and that's currently specific
to Zoom R16/24, handled in create_standard_audio_quirk().  Let's move
this also to the new quirk_flags.

Link: https://lore.kernel.org/r/20210729073855.19043-5-tiwai@suse.de


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