[ALSA] usb-audio: update quirk for Rane SL 1 (aka. Serato Scratch Live)

Allow the interface's mixer to be used, and give the interface its
correct name.

Signed-off-by: Mark Hills <mark@pogo.org.uk>
Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>