ALSA: usb-audio: Skip the clock selector inquiry for single connections

The current USB-audio driver gets an error at probing NUX MG-300 about
parsing the clocks.  This is because the firmware doesn't return the
proper connection of the clock selector that is connected to a single
clock; it's likely that the firmware was lazy^w optimized and the
inquiry wasn't handled.  Actually it makes little sense to inquire and
set up the single connection explicitly.

This patch fixes the issue by simply skipping the clock selector
inquiry if it's a single connection.

Reported-by: Mike Oliphant <oliphant@nostatic.org>
Link: https://lore.kernel.org/r/20210120213932.1971-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>