ALSA: usb-audio: Disable Lenovo P620 Rear line-in volume control

The USB device (0x17aa:0x1046) that support Lenovo P620 rear panel
line-in claim to support volume control, but it doens't seem to have an
AMP, so when line-in volume lowers below 80, nothing gets recorded
anymore.

Disable the volume control to workaround the issue.

Fixes: f8c11eb7 ("ALSA: usb-audio: Add support for Lenovo ThinkStation P620")
Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Link: https://lore.kernel.org/r/20200810133108.31580-1-kai.heng.feng@canonical.comSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/mixer_maps.c

@@ -371,6 +371,7 @@ static const struct usbmix_name_map asus_rog_map[] = {
 };
 
 static const struct usbmix_name_map lenovo_p620_rear_map[] = {
+	{ 19, NULL, 2 }, /* FU, Volume */
 	{ 19, NULL, 12 }, /* FU, Input Gain Pad */
 	{}
 };