ALSA: usb-audio: Fix mixer controls' USB interface for Kingston HyperX Amp (0951:16d8)

Use the USB interface of the mixer that the control
was created on instead of the default control interface.

This fixes the Kingston HyperX AMP (0951:16d8) which has
controls on two interfaces.

Signed-off-by: Chris Wulff <crwulff@gmail.com>
Link: https://lore.kernel.org/r/20200314165449.4086-2-crwulff@gmail.com


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