ALSA: usb-audio: Add mixer control for Digidesign Mbox 1 clock source

This patch provides the infrastructure for the Digidesign Mbox 1
to have a mixer control for selecting the clock source.
Valid options are Internal and S/PDIF external sync.
A non-documented command is sent to the device to enable this feature
found by reverse engineering and bus snooping.

Signed-off-by: Damien Zammit <damien@zamaudio.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/mixer_maps.c

@@ -179,6 +179,11 @@ static struct usbmix_name_map audigy2nx_map[] = {
 	{ 0 } /* terminator */
 };
 
+static struct usbmix_name_map mbox1_map[] = {
+	{ 1, "Clock" },
+	{ 0 } /* terminator */
+};
+
 static struct usbmix_selector_map c400_selectors[] = {
 	{
 		.id = 0x80,
@@ -415,6 +420,10 @@ static struct usbmix_ctl_map usbmix_ctl_maps[] = {
 		.id = USB_ID(0x0ccd, 0x0028),
 		.map = aureon_51_2_map,
 	},
+	{
+		.id = USB_ID(0x0dba, 0x1000),
+		.map = mbox1_map,
+	},
 	{
 		.id = USB_ID(0x13e5, 0x0001),
 		.map = scratch_live_map,