usb: Update struct usb_device to indicate speed enum

The speed member actually uses an enum, so add this to the comment.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Marek Vasut <marex@denx.de>

include/usb.h

@@ -103,7 +103,7 @@ enum {
  */
 struct usb_device {
 	int	devnum;			/* Device number on USB bus */
-	int	speed;			/* full/low/high */
+	enum usb_device_speed speed;	/* full/low/high */
 	char	mf[32];			/* manufacturer */
 	char	prod[32];		/* product */
 	char	serial[32];		/* serial number */