libpayload: Add enum for USB speeds

These values are already used in this usb stack.

Change-Id: If96f1dc2b67fbc13dfc4ae2d84e8f9945aa03163
Signed-off-by: Nico Huber <nico.huber@secunet.com>
Reviewed-on: http://review.coreboot.org/3448
Reviewed-by: Anton Kochkov <anton.kochkov@gmail.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

payloads/libpayload/include/usb/usb.h

@@ -93,6 +93,7 @@ typedef struct {
 	endpoint_type type;
 } endpoint_t;
 
+enum { FULL_SPEED = 0, LOW_SPEED = 1, HIGH_SPEED = 2, SUPER_SPEED = 3 };
 
 struct usbdev {
 	hci_t *controller;