ALSA: snd-usb: Accept UAC2 FORMAT_TYPE descriptors with bLength > 6

The Focusrite Scarlett 18i6 USB has them that way, which is probably a
bug. Anyway, the driver should simply ignore this fact.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Reported-by: Nicolai Krakowiak <nicolai.krakowiak@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>