ALSA: usb-audio: initialize variables that could ignore errors

ALSA: usb-audio: initialize variables that could ignore errors

clang static analysis reports this representative issue
mixer.c:1548:35: warning: Assigned value is garbage or undefined
        ucontrol->value.integer.value[0] = val;
                                         ^ ~~~

The filter_error() macro allows errors to be ignored.
If errors can be ignored, initialize variables
so garbage will not be used.

Fixes: 48cc42973509 ("ALSA: usb-audio: Filter error from connector kctl ops, too")
Signed-off-by: Tom Rix <trix@redhat.com>
Link: https://lore.kernel.org/r/20220126182142.1184819-1-trix@redhat.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>