ALSA: usb-audio: Skip generic sync EP parse for secondary EP

When ep_idx is already non-zero, it means usually a capture stream
that is set up explicity by a fixed-format quirk, and applying the
check for generic (non-implicit-fb) sync EPs might hit incorrectly,
resulting in a bogus sync endpoint for the capture stream.

This patch adds a check for the ep_idx and skip if it's a secondary
endpoint.  It's a part of the fixes for regressions on 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/20220606160910.6926-2-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/pcm.c

@@ -304,7 +304,7 @@ int snd_usb_audioformat_set_sync_ep(struct snd_usb_audio *chip,
 	 * Generic sync EP handling
 	 */
 
-	if (altsd->bNumEndpoints < 2)
+	if (fmt->ep_idx > 0 || altsd->bNumEndpoints < 2)
 		return 0;
 
 	is_playback = !(get_endpoint(alts, 0)->bEndpointAddress & USB_DIR_IN);