ALSA: usb-audio: work around KEF X300A firmware bug

commit 624aef49 upstream.

When the driver tries to access Function Unit 10, the KEF X300A
speakers' firmware apparently locks up, making even PCM streaming
impossible.  Work around this by ignoring this FU.
Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

sound/usb/mixer_maps.c

@@ -304,6 +304,11 @@ static struct usbmix_name_map hercules_usb51_map[] = {
 	{ 0 }				/* terminator */
 };
 
+static const struct usbmix_name_map kef_x300a_map[] = {
+	{ 10, NULL }, /* firmware locks up (?) when we try to access this FU */
+	{ 0 }
+};
+
 /*
  * Control map entries
  */
@@ -371,6 +376,10 @@ static struct usbmix_ctl_map usbmix_ctl_maps[] = {
 		.map = scratch_live_map,
 		.ignore_ctl_error = 1,
 	},
+	{
+		.id = USB_ID(0x27ac, 0x1000),
+		.map = kef_x300a_map,
+	},
 	{ 0 } /* terminator */
 };