ALSA: usb-audio: Add implicit_fb module option

A new module option, implicit_fb, is added to specify the driver
looking for the implicit feedback sync.  This can be useful for a
device that could be working better in the implicit feed back mode and
user wants to test it quickly.  When this works, we can add the quirk
entry easily.

Tested-by: Keith Milner <kamilner@superlative.org>
Tested-by: Dylan Robinson <dylan_robinson@motu.com>
Link: https://lore.kernel.org/r/20201123085347.19667-40-tiwai@suse.de


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