ALSA: usb-audio: Set up (implicit) sync for Saffire 6

Focusrite Saffire 6 has fixed audioformat quirks with multiple
endpoints assigned to a single altsetting.  Unfortunately the generic
parser couldn't detect the sync endpoint correctly as the implicit
sync due to the missing EP attribute bits.  In the former kernels, it
used to work somehow casually, but it's been broken for a while after
the large code change in 5.11.

This patch cures the regression by the following:
- Allow the static quirk table to provide the sync EP information;
  we just need to fill the fields and let the generic parser skipping
  parsing if sync_ep is already set.
- Add the sync endpoint information to the entry for Saffire 6.

Fixes: 7b0efea4

 ("ALSA: usb-audio: Add missing ep_idx in fixed EP quirks")
Reported-and-tested-by: André Kapelrud <a.kapelrud@gmail.com>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20220606160910.6926-3-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>