USB: gadget: composite: Add userspace notifications for USB state changes

Add switch to notify current USB configuration.  This can be used to detect
USB connect and disconnect events.

Broadcast a change via the usb_composite class when a USB function is
enabled or disabled.

Rename usb_function.hidden to usb_function.disabled.

Signed-off-by: Mike Lockwood <lockwood@android.com>
diff --git a/drivers/usb/gadget/f_rndis.c b/drivers/usb/gadget/f_rndis.c
index 3b062d9..c0c3280 100644
--- a/drivers/usb/gadget/f_rndis.c
+++ b/drivers/usb/gadget/f_rndis.c
@@ -864,7 +864,7 @@
 
 #ifdef CONFIG_USB_ANDROID_RNDIS
 	/* start disabled */
-	rndis->port.func.hidden = 1;
+	rndis->port.func.disabled = 1;
 #endif
 
 	status = usb_add_function(c, &rndis->port.func);