usb: provide typedefs for all the function callbacks.

This makes it easier to read for most people, and makes it substantially
easier to review changes in the function signatures themselves at a
later date.

lib/usb/usb.c

@@ -143,8 +143,7 @@ void usbd_disconnect(usbd_device *usbd_dev, bool disconnected)
 }
 
 void usbd_ep_setup(usbd_device *usbd_dev, uint8_t addr, uint8_t type,
-		   uint16_t max_size,
-		   void (*callback)(usbd_device *usbd_dev, uint8_t ep))
+		   uint16_t max_size, usbd_endpoint_callback callback)
 {
 	usbd_dev->driver->ep_setup(usbd_dev, addr, type, max_size, callback);
 }