ALSA: usb-audio: Fix a missing error check in scarlett gen2 mixer

ALSA: usb-audio: Fix a missing error check in scarlett gen2 mixer

The check of the returned error code is missing in
scarlett2_update_monitor_other().  Let's fix it.

Fixes: d5bda7e03982 ("ALSA: usb-audio: scarlett2: Add support for the talkback feature")
Reported-by: kernel test robot <lkp@intel.com>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/202109131831.9IodEzRx-lkp@intel.com
Link: https://lore.kernel.org/r/20210929073540.9611-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>