PATCH: ALSA USB MIDI: Fix case when USB MIDI interface has more than one extra endpoint descriptor

[Andreas Steinmetz <ast@domdv.de>]

The Miditech MIDIFACE 16x16 (USB ID 1290:1749) has more than one extra
endpoint descriptor.

The first extra descriptor is: 0x06 0x30 0x00 0x00 0x00 0x00

As the code in snd_usbmidi_get_ms_info() looks only at the
first extra descriptor to find USB_DT_CS_ENDPOINT the device
as such is recognized but there is neither input nor output
configured.

The patch iterates through the extra descriptors to find the
proper one. With this patch the device is correctly configured.

Signed-off-by: Andreas Steinmetz <ast@domdv.de>

--- a/sourd/usb/midi.c	2020-03-26 17:34:40.073929497 +0100
+++ b/sound/usb/midi.c	2020-03-26 17:43:39.123252612 +0100
@@ -1839,7 +1839,8 @@
 	struct usb_host_endpoint *hostep;
 	struct usb_endpoint_descriptor *ep;
 	struct usb_ms_endpoint_descriptor *ms_ep;
-	int i, epidx;
+	int i, epidx, extralen;
+	unsigned char *extra;
 
 	intf = umidi->iface;
 	if (!intf)
@@ -1863,12 +1864,21 @@
 		ep = get_ep_desc(hostep);
 		if (!usb_endpoint_xfer_bulk(ep) && !usb_endpoint_xfer_int(ep))
 			continue;
-		ms_ep = (struct usb_ms_endpoint_descriptor *)hostep->extra;
-		if (hostep->extralen < 4 ||
+		extralen = hostep->extralen;
+		extra = hostep->extra;
+repeat:		ms_ep = (struct usb_ms_endpoint_descriptor *)extra;
+		if (extralen < 1)
+			continue;
+		if (extralen < 4 ||
 		    ms_ep->bLength < 4 ||
 		    ms_ep->bDescriptorType != USB_DT_CS_ENDPOINT ||
-		    ms_ep->bDescriptorSubtype != UAC_MS_GENERAL)
-			continue;
+		    ms_ep->bDescriptorSubtype != UAC_MS_GENERAL) {
+			if (!extra[0])
+				continue;
+			extralen -= extra[0];
+			extra += extra[0];
+			goto repeat;
+		}
 		if (usb_endpoint_dir_out(ep)) {
 			if (endpoints[epidx].out_ep) {
 				if (++epidx >= MIDI_MAX_ENDPOINTS) {