ALSA: usb-audio: Avoid doubly initialization for implicit fb

The implicit feedback mode initializes both the main data stream and
the sync data stream.  When a sync stream was already opened, this
would result in the doubly initialization and might screw up things.

Add the check of already opened sync streams and skip the unnecessary
initialization.

Tested-by: Keith Milner <kamilner@superlative.org>
Tested-by: Dylan Robinson <dylan_robinson@motu.com>
Link: https://lore.kernel.org/r/20201123085347.19667-14-tiwai@suse.de


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