ALSA: usb-audio: Fix Scarlett 6i6 initialization typo

The num_controls field was incorrectly set to 0 causing 6i6 to not be
initialized. Set this to 9.

Reported-and-tested-by: Mark Roberts <sunifiram@gmail.com>
Signed-off-by: Chris J Arges <chris.j.arges@canonical.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>