ALSA: usb-audio: Avoid multiple assignments in mixer_quirks

Handle report from checkpatch.pl:

  CHECK: multiple assignments should be avoided

Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@collabora.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://patch.msgid.link/20250526-dualsense-alsa-jack-v1-6-1a821463b632@collabora.com