ALSA: usb-audio: Show sync endpoint information in proc outputs

Now the sync endpoints have been parsed at the beginning and won't be
changed dynamically, let's show them in the proc outputs for helping
debugging.

Tested-by: Keith Milner <kamilner@superlative.org>
Tested-by: Dylan Robinson <dylan_robinson@motu.com>
Link: https://lore.kernel.org/r/20201123085347.19667-36-tiwai@suse.de


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