USB: gadget: composite: Dynamically set IAD bFirstInterface

bFirstInterface should be set equal to bInterfaceNumber of
the first interface in a grouping. It's currently being set
only when a composite device is bound, and it does not get
updated when functions are enabled or disabled.

Signed-off-by: Mike Lockwood <lockwood@android.com>
diff --git a/drivers/usb/gadget/composite.c b/drivers/usb/gadget/composite.c
index c2684b5..96d8987 100644
--- a/drivers/usb/gadget/composite.c
+++ b/drivers/usb/gadget/composite.c
@@ -305,6 +305,7 @@
 {
 	struct usb_config_descriptor	*c = buf;
 	struct usb_interface_descriptor *intf;
+	struct usb_interface_assoc_descriptor *iad = NULL;
 	void				*next = buf + USB_DT_CONFIG_SIZE;
 	int				len = USB_BUFSIZ - USB_DT_CONFIG_SIZE;
 	struct usb_function		*f;
@@ -358,6 +359,20 @@
 					intf->bInterfaceNumber = interfaceCount++;
 				else
 					intf->bInterfaceNumber = interfaceCount - 1;
+				if (iad) {
+					iad->bFirstInterface =
+							intf->bInterfaceNumber;
+					iad = NULL;
+				}
+			} else if (intf->bDescriptorType ==
+					USB_DT_INTERFACE_ASSOCIATION) {
+				/* This will be first if it exists. Save
+				 * a pointer to it so we can properly set
+				 * bFirstInterface when we process the first
+				 * interface.
+				 */
+				iad = (struct usb_interface_assoc_descriptor *)
+						dest;
 			}
 			dest += intf->bLength;
 		}